1H-Indole-3-carbaldehyde azine

The molecule of the title compound, C18H14N4, lies on a center of inversion such that there is one half-molecule in the asymmetric unit. The N—N single bond adopts a trans configuration and the indole fused-ring system is nearly coplanar with the –CH=N—N=CH– fragment [dihedral angle = 9.8 (2)°]. Adjacent molecules are linked by indole–azine N—H⋯N hydrogen bonds into a layer motif.

The molecule of the title compound, C 18 H 14 N 4 , lies on a center of inversion such that there is one half-molecule in the asymmetric unit. The N-N single bond adopts a trans configuration and the indole fused-ring system is nearly coplanar with the -CH N-N CH-fragment [dihedral angle = 9.8 (2) ]. Adjacent molecules are linked by indoleazine N-HÁ Á ÁN hydrogen bonds into a layer motif.

Comment
Azines are readily synthesized by condensing hydrazine with an aldehyde; the crystal structures of a large number of substituted benzaldehdye azines have been reported. The structure of the parent aromatic compound, benzalazine, has been known for a long time (Burke-Laing & Laing, 1976;Mom & de With, 1978;Sinha, 1970). There are few examples of heterocyclic azines, and their rarity can be attributed to the difficulty of synthesizing the starting aldehyde reactant. Among the few are, for example, unsubstituted and methyl-subsituted thiophene-2-aldehyde azine (Lin et al., 2001a(Lin et al., , 2001b) and a pyrrole derivative has recently been reported (Wu et al., 2006).
3-Indole azine has been known for some time; it was first synthesized from indole-3-carboxaldehyde and hydrazine in order to examine its psychopharmacological activity (Alemany et al., 1970;Swaminathan Narasimhan, 1964). The title compound was the unexpected decomposition product of the Schiff base derived from the condensation of carbohydrazide and indole-3-carboxaldehyde. The molecule (Scheme I, Fig. 1) lies about a center-of-inversion such that there is half a molecule in the asymmetric unit. The N-N single-bond adopts a trans configuration and the indolyl fused-ring is nearly coplanar with the -CH=N-N=CH-fragment. Adjacent molecules are linked by an N-H indole ···N azine hydrogen bonds into layer motif (Fig. 2).

Experimental
The reaction of carbohydrazide (0.3 g, 3.3 mmol) and indole −3-carboxaldehyde (1 g, 6.6 mmol) in ethanol under reflux for 2 h gave the corresponding Schiff base. This compound (0.2 g, 0.6 mmol), zinc acetate (0.06 g,0.3 mmol) and several drops of triethylamine were dissolved in 10 ml e thanol. The contents were heated in a 25-ml, stainless-steel Paar bomb for for 2 d at 373 K. The bomb was cooled to room temperature over several hours. Well formed crystals were isolated from the cooled bomb.

Refinement
Carbon-bound H-atoms were placed in calculated positions (C-H 0.93 Å) and were included in the refinement in the riding model approximation, with U(H) set to 1.2U(C). The amino H-atom was located in a difference Fourier map, and was freely refined. Fig. 1. Displacement ellipsoid plot of (I) at the 50% probability level. H atoms are drawn as spheres of arbitrary radiius.