Crystal structure of a new monoclinic polymorph of N-(4-methylphenyl)-3-nitropyridin-2-amine

Molecules in both polymorphs of the title compound display deviations from planarity owing to crystal packing effects.


Chemical context
Original interest in the molecules related to the title compound revolved around their fluorescence properties (Kawai et al., 2001;Abdullah, 2005). The title compound was isolated during an ongoing study of co-crystals formed between carboxylic acids and pyridine-containing molecules (Arman & Tiekink, 2013;Arman et al., 2014), designed to prove the robustness of the {Á Á ÁHOC( O)Á Á ÁN(pyridine)} heterosynthon in co-crystals (Shattock et al., 2008) of functionalized carboxylic acids with pyridine derivatives. The crystal structure of the title compound has been reported previously as a monoclinic (P2 1 /c, with Z 0 = 2) polymorph (Akhmad Aznan et al., 2010), and the present polymorph was isolated from a failed co-crystallization experiment as detailed in Section 5. The phenomenon of isolating polymorphs from co-crystallization experiments is gaining increasing prominence, especially since the isolation of a second form of aspirin (Vishweshwar et al., 2005), and led Zaworotko to suggest cocrystallization experiments should also be employed in polymorph screening (Arora & Zaworotko, 2009).

Structural commentary
Four crystallographically independent molecules comprise the asymmetric unit (Fig. 1). Each molecule features a secondary ISSN 1600-5368 amine linking nitrobenzene and tolyl groups, with the nitropyridyl N atom syn to the toluene ring. An intramolecular N-HÁ Á ÁO hydrogen bond closes an S(6) loop in each molecule (Table 1). This feature of the structure confers coplanarity of the nitro group with the pyridyl ring to which it is attached; the maximum deviation from coplanarity is seen in the pyridyl/ nitro group dihedral angle of 5.2 (3) , for the N10-containing molecule. More significant differences are found in the dihedral angles between the two rings, i.e. 23.79 (19), 26.24 (19), 6.57 (18) and 2.92 (19) for the N1-, N4-, N7-and N10containing molecules, respectively. Similar conformations were observed for the two independent molecules in the previously reported P2 1 /c polymorph (Aznan Akhmad et al., 2010). Here, the dihedral angles between the rings were 17.42 (16) and 34.64 (16) , resembling the N1-and N4containing molecules in the present study rather than the almost planar N7-and N10-containing molecules.

Figure 2
Overlay diagram of conformations of the title compound. The N1-, N4-, N7-and N10-containing molecules determined in the present study are shown in red, pink, blue and aqua, respectively; the N1-, N7-and N10containing molecules were inverted for a better fit. The green and yellow images correspond to the unique molecules in the known polymorph and the black image corresponds to the geometry-optimized structure.

Figure 5
Unit-cell contents shown in projection down the a axis. The C-HÁ Á ÁO, C-HÁ Á Á, N-OÁ Á Á andcontacts are shown as orange, brown, blue and purple dashed lines, respectively.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Carbon-bound H atoms were placed in calculated positions (C-H = 0.95 Å ) and were included in the refinement in the riding-model approximation, with U iso (H) set at 1.2U eq (C). N-bound H atoms were located in a difference Fourier map but were refined with a distance restraint of N-H = 0.88AE0.01 Å and with U iso (H) set at 1.2U eq (N). In the absence of significant anomalous scattering effects, 4208 Friedel pairs were averaged in the final refinement. Computer programs: CrysAlis PRO (Agilent, 2013), SHELXS97 and SHELXL97 (Sheldrick, 2008), ORTEP-3 for Windows (Farrugia, 2012), QMol (Gans & Shalloway, 2001) and DIAMOND (Brandenburg, 2006), publCIF (Westrip, 2010 (Agilent, 2013); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012), QMol (Gans & Shalloway, 2001) and DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010). 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 cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s involving l.s. planes. 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 R-factors(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.