Crystal structures of 2-(4-nitrophenyl)-3-phenyl-2,3-dihydro-4H-1,3-benzothiazin-4-one and 2-(2-nitrophenyl)-3-phenyl-2,3-dihydro-4H-1,3-benzothiazin-4-one

In the crystal structures of the racemic para and ortho isomers of 2-(4-nitrophenyl)-3-phenyl-2,3-dihydro-4H-1,3-benzothiazin-4-one, the six-membered thiazine ring of the benzothiazone parent molecule displays a screw-boat conformation and a near-screw-boat conformation, respectively. In the crystals of both isomers, weak C—H⋯O hydrogen-bonding interactions give rise to one-dimensional structures.

Here we report the synthesis and crystal structures of the para-and ortho-nitro analogs of C 20 H 14 N 2 O 3 S, the title ISSN 2056-9890 compounds, 2-(4-nitrophenyl)-3-phenyl-2,3-dihydro-4H-1,3benzothiazin-4-one, (I) and (II), respectively, completing the set and allowing for comparison of the structural effects of the differently positioned nitro substituent groups.

Structural commentary
The crystal structures of the two racemic isomers (I) and (II) show some differences and some similarities among them-selves as well as with the meta-form (III) reported earlier (Yennawar et al., 2013). The para-nitro form (I) (Fig. 1) is triclinic, space group P1, while the ortho-nitro form (II) (Fig. 2) is monoclinic, space group P2 1 /n, as was the meta-form. The structures show screw-boat (I) or near screw-boat (II) conformations for the thiazine ring, as compared to an envelope conformation in the meta-form (III) and the unsubstituted 2,3-diphenyl compound (IV). In both (I) and (II), the three phenyl-ring planes are close to orthogonal with each other, with dihedral angles between the planes of the two substituent groups (C9-C14 = 4-nitrophenyl ring and C15-C20 = phenyl ring) with the benzene ring (C3-C8) of the parent benzothiazine moiety of 75.93 (5)  Molecular conformation and atom-numbering scheme for (I). Displacement ellipsoids are drawn at the 50% probability level Molecular conformation and atom-numbering scheme for (II). Displacement ellipsoids are drawn at the 50% probability level. Table 1 Hydrogen-bond geometry (Å , ) for (I).

Figure 3
Crystal packing in (I) showing intermolecular hydrogen-bonding interactions as dashed lines.

Supramolecular features
In (I), as in the meta-form (Yennawar et al., 2013), one of the O atoms of the nitro group accepts a weak aromatic C20-H20Á Á ÁO3 i hydrogen bond (Table 1), forming a large centrosymmetric cyclic dimer through an R 2 2 (22) association. A further set of weak inversion-related C14-H14Á Á ÁO1 ii interactions with carbonyl O-atom acceptors give a second cyclic dimer [graph set R 2 2 (14)], forming a zigzag chain structure extending along c (Fig. 3). In (II), a weak intermolecular C17-H17Á Á ÁO1 iii hydrogen bond to the thiazinone O-atom acceptor ( Table 2) gives rise to a chain extending along the b-axis direction (Fig. 4). In addition, C-HÁ Á Á interactions are present in both (I) ( Table 1) and (II) ( Table 2) [minimum CÁ Á Áring-centroid separations of 3.630 (2) and 3.581 (2) Å , respectively], linking the chains to form sheets in the bc plane in (I) and a three-dimensional structure in (II). There are no other significant interactions present in either structure.

Figure 4
Crystal packing in (II) showing intermolecular hydrogen-bonding interactions as dashed lines.

Synthesis and crystallization
The syntheses were achieved in the manner previously reported, by condensation of thiosalicylic acid with a diaryl imine (Yennawar et al., 2013(Yennawar et al., , 2014, as follows: A two-necked 25 ml round-bottomed flask was oven-dried, cooled under N 2 , and charged with a stir bar and the imine (6 mmol). Tetrahydrofuran (2.3 ml) was added, the solid dissolved, and the solution was stirred. Pyridine (1.95 ml, 24 mmol) was added after which thiosalicylic acid (0.93 g, 6 mmol) was added. Finally, 2,4,6-tripropyl-1,3,5,2,4,6-trioxatriphosphorinane 2,4,6-trioxide (T3P) in 2-methyltetrahydrofuran (50% w/w; 7.3 ml, 12 mmol) was added. The reaction was stirred at room temperature and followed by TLC. The mixture was poured into a separatory funnel with dichloromethane and distilled water. The layers were separated and the aqueous fraction was then extracted twice with dichloromethane. The organic fractions were combined and washed with saturated aqueous solutions of sodium bicarbonate and then saturated sodium chloride. The organic fraction was dried over sodium sulfate and concentrated under vacuum. The crude solid was chromatographed on 30 g flash silica gel and then recrystallized as described below.

(I) 2-(4-Nitrophenyl)-3-phenyl-2,3-dihydro-4H-1,3-benzothiazin-4-one
Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'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 > σ(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.  (7) 0.0347 (6) (7) −0.0120 (7) −0.0044 (6) C20 0.0447 (7) 0.0478 (7) 0.0372 (7) 0.0013 (6)    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 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.