2-Allyl-7-nitro-2H-indazole

The asymmetric unit of the title compound, C10H9N3O2, contains two independent molecules linked by a C—H⋯N hydrogen bond. Each molecule has a similar conformation, being built up from fused five- and six-membered rings, each linked to an ally and nitro group, respectively. The indazole ring system makes dihedral angles of 2.7 (2) and 2.2 (2)°, respectively, with the plane through the nitro group. The allyl group is nearly perpendicular to the indazole system, as indicated by the N—N—C—C torsion angles of −75.3 (2) and −82.2 (2)°, this being the most important difference between the conformations of the two molecules. In the crystal, molecules are linked by C—H⋯O and π–π [inter-centroid distance = 3.6225 (8) Å] interactions to form a three-dimensional network.


Comment
The indazole subunit in organic molecules is an important structure in many drug substances with a wide range of pharmacological effects: e.g., anti-tumor, anti-microbial, anti-platelet, anti-HIV, and anti-inflammatory (Baraldi et al., 2001;Li et al., 2003;Lee et al., 2001;Rodgers et al., 1996;Schmidt et al., 2008). The present work is a continuation of the investigation of the indazole derivatives published recently by our team (El Brahmi et al., 2012;Chicha et al., 2013).
The plot of the structure of the title compound, with two molecules in the asymmetric unit, shows them linked by a C8 -H8B···N4 hydrogen bond, Fig. 1. In the molecules, the allyl groups are nearly perpendicular to indazole planes as indicated by the torsion angles of C8-C9-N1-N2 = -75.3 (2)° and C18-C19-N4-N5 = -82.2 (2)°. This is the most important difference between the two conformations of the molecules as shown in the overlay diagram of the two crystallographically independent molecules (Fig. 2). The dihedral angles of 2.7 (2) and 2.2 (2)°, respectively, between the fused ring systems and the nitro groups lead to a synperiplanar conformation for each molecule.

Experimental
To a solution of 7-nitroindazole (6.13 mmol) in acetone (15 ml) was added potassium hydroxide (6.8 mmol). After 15 min. at 298 K, allyl bromide (12.26 mmol) was added drop wise. Upon disappearance of the starting material as indicated by TLC, the resulting mixture was evaporated. The crude material was dissolved with EtOAc (50 ml), washed with water and brine, dried over MgSO 4 and the solvent was evaporated in vacuo. The resulting residue was purified by column chromatography (EtOAc/hexane 3/7). The title compound was recrystallized from ethanol at room temperature giving colorless crystals (m.p. 358 (1) K, yield: 65%).

Refinement
H atoms were located in a difference map and treated as riding with C-H = 0.93-0.97 Å, and with U iso (H) = 1.2U eq .

Figure 2
Overlay diagram of the two crystallographically independent molecules highlighting the different orientations of the allyl groups.  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.