3-[2-(4,4-Dimethyl-2,6-dioxocyclohexylidene)hydrazinyl]benzonitrile

The title compound, C15H15N3O2, contains benzonitrile and 4,4-dimethyl-2,6-dioxocyclohexylidene groups connected via a hydrazinyl group. The structure is in the hydrazone tautomeric form in the solid state. The benzonitrile and hydrazinyl groups (3-hydrazinylbenzonitrile) are essentially coplanar with an r.m.s. deviation of 0.016 Å. Intramolecular N—H⋯O hydrogen bonding helps to stabilize the molecular structure, and weak intermolecular C—H⋯O hydrogen bonding is present in the crystal structure.

Azo dyes are known to exist in the azo-hydrazone tautomeric forms (Saylam et al., 2008;Seferoğlu et al., 2008;Seferoğlu et al., 2009). The dyes may exist in two possible tautomeric forms, namely azo form A and hydrazone form B as depicted in Figure 3. It is suggested that in a real azo compound the N=N double bond should have a length of 1.20-1.28 Å and the bond length of N-N single bonds, as in hydrazone tautomers, should be more than 1.4 Å (Kelemen et al., 1982). In the title compound, N-N bond length is 1.304 Å, between the suggested N=N double bond and N-N single bond lengths. The bond lengths of N(sp3)-C(sp2), N(sp2)-C(sp2) and N(sp3)-N(sp2) in related hydrazone tatutomers are listed in Table 2.

Experimental
A hydrochloric acid solution (2.5 ml) of 3-aminobenzonitrile (1.18 g, 0.010 mol) and an aqueous solution (10 ml) of sodium nitrite (0.69 g, 0.010 mol) were mixed and stirred at 273 K for 1 h, followed by the addition of ethanol solution (10 ml) of the coupling component 5,5-dimethylcyclohexane-1,3-dione (1.40 g, 0.010 mol) and continued stirring at 273 K for 4 h. The resulting product was filtered and washed with water, dried, and crystallized from ethanol gave fine crystals of benzonitrile,

Refinement
H atoms attached to carbon atoms were placed in calculated positions with U iso (H) = 1.2U eq (C). The coordinates of the amine hydrogen obtained from a difference map and refined isotropically.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 Rfactors(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.