2,2′-(Propane-1,3-diyl)bis(2H-indazole)

The title molecule, C17H16N4, is a bis-indazole crystallized in the rare 2H-tautomeric form. Indazole heterocycles are connected by a propane C3 chain, and the molecule is placed on a general position, in contrast to the analogous compound with a central C2 ethane bridge, which was previously found to be placed on an inversion center in the same space group. In the title molecule, indazole rings make a dihedral angle of 60.11 (7)°, and the bridging alkyl chain displays a trans conformation, resulting in a W-shaped molecule. In the crystal, molecules interact weakly through π–π contacts between inversion-related pyrazole rings, with a centroid–centroid separation of 3.746 (2) Å.

The title molecule, C 17 H 16 N 4 , is a bis-indazole crystallized in the rare 2H-tautomeric form. Indazole heterocycles are connected by a propane C 3 chain, and the molecule is placed on a general position, in contrast to the analogous compound with a central C 2 ethane bridge, which was previously found to be placed on an inversion center in the same space group. In the title molecule, indazole rings make a dihedral angle of 60.11 (7) , and the bridging alkyl chain displays a trans conformation, resulting in a W-shaped molecule. In the crystal, molecules interact weakly throughcontacts between inversion-related pyrazole rings, with a centroid-centroid separation of 3.746 (2) Å .
The title compound was prepared through a three steps procedure, the key step being the cyclization of a nitroamine derivative (compound P, see Fig. 1 and Experimental). The resulting bis-indazole consists of two 2H-indazole heterocycles connected by an alkyl C 3 bridge (Fig. 2). The molecule lies in a general position, in space group Pbca. It is worth noting that the analogue compound with a central C 2 bridge, for which we reported the X-ray structure (Rodríguez de Barbarín et al., 2006), was found to crystallize in the same space group, although the molecule was placed on an inversion center. From this pair of structures now determined, we can propose the following general rule for n-alkyl bridged bis(2H-indazoles): even-alkyl compounds should be centrosymmetric, while odd-alkyl derivatives are expected to be non-centrosymmetric, as the title compound.

Experimental
The title ligand, PI, was obtained by a three steps reaction procedure (Fig. 1). The condensation between 1,3-diaminopropane and 2-nitrobenzaldehyde produced the corresponding imine. Selective reduction of imine bonds with sodium borohydride in methanol gave amine P, which was isolated. Then, 0.044 g of Pd/C was added to a solution of P (0.005 mol) in ethanol.
This mixture was refluxed for 5.5 h, with addition of hydrazine monohydrate (0.110 mol) during the first 3 h. The mixture was filtered, distilled, and the organic phase was extracted. The product was purified by column chromatography with silica gel and ethyl acetate:hexane (2:1) as eluent.

Refinement
All H atoms were placed in idealized positions and refined as riding to their parent C atoms, with bond lengths fixed to 0.97 (methylene CH 2 ) or 0.93 Å (aromatic CH). Isotropic displacement parameters for H atoms were calculated as U iso (H) = 1.2 U eq (carrier atom). Fig. 1. Synthetic route for the title compound. P is the key intermediate and PI is the title compound. Fig. 2. ORTEP-like view of the title molecule, with displacement ellipsoids at the 30% probability level for non-H atoms. Fig. 3. A part of the crystal structure representing two molecules related by inversion, which interact through a π-π contact involving pyrazole rings (dashed line).