Crystal structure of (E)-1,3-dimethyl-2-[3-(3-nitrophenyl)triaz-2-en-1-ylidene]-2,3-dihydro-1H-imidazole

The title molecule, a rare example of a π-conjugated triazene, crystallized with two independent molecules in the asymmetric unit. In the crystal, the two independent molecules stack head-to-tail probably due to the presence of the dipole moment of the meta nitro group.


Chemical context
Triazenes are compounds containing three contiguous nitrogen atoms in a linear format with a double bond between the first and second N atoms; i.e., -N=N-N-. The structure of the triazene moiety is influenced by the resonance arising from delocalization of the electron lone-pair on the third N atom, towards the double bond. Triazenes are relatively old compounds from the organic chemist's viewpoint. It was as early as 1862 that Griess described a suitable method for the synthesis of 1,3-diphenyltriazene (Griess, 1862). At that time, no applications for triazenes could be found and these compounds were ignored for many decades. Unsubstituted triazenes are unstable under normal conditions; however, substituted triazenes are normally thermally stable. More recently, attention has been paid to substituted triazenes, especially to 1-aryl-3,3-dialkyl-triazenes [which were synthesized for the first time by Baeyer & Jaeger (1875)] because some of them show activity as insecticides (Giraldi et al., 1990). Currently, triazenes have found uses as alkylating agents in tumor therapy (Rouzer et al., 1996), as iodo-masking groups in the synthesis of small (Nicolaou et al., 1999) and macromolecules (Jones et al., 1997), and in the preparation of N-containing heterocycles (Wirshun et al., 1998). The first report on a -conjugated triazenes was by Winberg et al. (1965), and more recently, we have reported the syntheses and structures of a variety of such -conjugated triazenes (Patil et al., 2014).

Structural commentary
The molecular structures of the two independent molecules (A and B) of the title compound are illustrated in Fig. 1. Both molecules have an E conformation about the -N5 N4-and -N11 N10-bonds and the bond lengths and angles of theconjugated triazene unit (Table 1) are very similar to those in related structures (Khramov & Bielawski, 2005, 2007Jishkariani et al., 2013;Tennyson et al., 2010). The two molecules have slightly twisted overall conformations, with the imidazole ring (N1/N2/C1-C3) inclined to the benzene ring (C6-C11) by 8.12 (4) in molecule A, while in molecule B the two rings (N7/ N8/C12-C14 and C17-C22) are inclined to one another by 7.73 (4) .

Supramolecular features
In the crystal, the independent molecules are linked by C-HÁ Á ÁO hydrogen bonds forming -A-A-A-and -B-B-Bchains along [100]. The chains are linked by C-HÁ Á ÁO and C-HÁ Á ÁN hydrogen bonds, forming sheets lying parallel to (001); see Fig. 2 and Table 2. The sheets are linked by further C-HÁ Á ÁN hydrogen bonds and C-HÁ Á Á andinteractions [Cg1Á Á ÁCg4 i = 3.5243 (5) Å ; Cg1 and Cg4 are the centroids of the imidazole ring of molecule A and the benzene ring of molecule B; symmetry code: (i) x, y, z À 1], forming a three-dimensional framework structure ( Fig. 3  A view of the molecular structure of the two independent molecules (A and B) of the title compound, with atom labelling. Displacement ellipsoids are drawn at the 50% probability level.  Hydrogen-bond geometry (Å , ).

Figure 2
A view along the c axis of the crystal packing of title compound, with hydrogen bonds shown as dashed lines (see Table 2 for details).

Figure 3
A view along the a axis of the crystal packing of the title compound, with hydrogen bonds shown as dashed lines (see Table 2 for details; H atoms not involved in hydrogen bonds have been omitted for clarity).

Database survey
The first synthesis of a -conjugated triazene was reported on in 1965 (Winberg & Coffman, 1965). The first X-ray structure analysis of a -conjugated triazene appeared many years later (Khramov et al., 2005). A search of the WebCSD database, gave 15 hits for -conjugated triazenes. Two of these structures (Patil et al., 2014) employed 1,3-dimethylimidazolium iodide as the carbene precursor. Although, there is a compound that closely resembles the title compound in the literature (Patil et al., 2014), it differs in the position of the nitro-substituent in the aromatic moiety. In the title compound, the nitro substituent is in the meta position, while the parallel report has the nitro substituent in the para position.

Synthesis and crystallization
1-Azido-3-nitrobenzene was prepared according to the literature procedure (Siddiki et al., 2013). The synthesis of 1,3dimethylimidazolium iodide was carried out accordingly to literature procedure (Oertel et al., 2011). For the synthesis of the title compound, 1-azido-3-nitrobenzene (196 mg, 1.2 mmol) was added in one portion to a suspension of 1,3dimethylimidazolium iodide (134 mg, 0.6 mmol) in dry THF (5 mL) and stirred at room temperature for 5 min. In one portion, NaH (24 mg, 0.6 mmol, 60% in mineral oil) was added to the reaction vessel and the resulting mixture was stirred at room temperature for 6 h. The yellowish-orange precipitate that formed was collected by filtration and dried under reduced pressure, giving the title compound as an orange crystalline solid (yield 140 mg, 90%