2-{(E)-[(2Z)-2-(1,2-Dihydrophthalazin-1-ylidene)hydrazinylidene]methyl}phenol

The title compound, C15H12N4O, adopts an E conformation with respect to the azomethine bond and crystallizes in its hydrazinylidene tautomeric form. The dihedral angle between the ring systems is 15.98 (7)°. The phenol O—H group forms an intramolecular O—H⋯N hydrogen bond. In the crystal, pairs of N—H⋯N and C—H⋯O hydrogen bonds link neighbouring molecules into centrosymmetric dimers. These dimers are interconnected by means of three types of π–π stacking interactions. One, with a centroid–centroid distance of 3.577 (1) Å [interplanar separation = 3.4673 (6) Å], connects adjacent molecules into centrosymmetric dimers. The other two interactions, on the outward facing sides of the dimers, are between phenol rings of neighboring molecules [centroid–centroid separation = 3.7907 (13) Å and interplanar separation = 3.5071 (8) Å], and between phthalazin units [centroid–centroid separation = 3.6001 (12) Å and interplanar separation = 3.4891 (7) Å]. In combination, the π–π interactions lead to the formation of infinite layers with molecules stacked along [0-11]. These layers are, in turn, connected with neighbouring layers through the N—H⋯N and C—H⋯O hydrogen bonds, yielding a three-dimensional supramolecular architecture.

The title compound, C 15 H 12 N 4 O, adopts an E conformation with respect to the azomethine bond and crystallizes in its hydrazinylidene tautomeric form. The dihedral angle between the ring systems is 15.98 (7) . The phenol O-H group forms an intramolecular O-HÁ Á ÁN hydrogen bond. In the crystal, pairs of N-HÁ Á ÁN and C-HÁ Á ÁO hydrogen bonds link neighbouring molecules into centrosymmetric dimers. These dimers are interconnected by means of three types ofstacking interactions. One, with a centroid-centroid distance of 3.577 (1) Å [interplanar separation = 3.4673 (6) Å ], connects adjacent molecules into centrosymmetric dimers. The other two interactions, on the outward facing sides of the dimers, are between phenol rings of neighboring molecules [centroid-centroid separation = 3.7907 (13) Å and interplanar separation = 3.5071 (8) Å ], and between phthalazin units [centroid-centroid separation = 3.6001 (12) Å and interplanar separation = 3.4891 (7) Å ]. In combination, theinteractions lead to the formation of infinite layers with molecules stacked along [011]. These layers are, in turn, connected with neighbouring layers through the N-HÁ Á ÁN and C-HÁ Á ÁO hydrogen bonds, yielding a three-dimensional supramolecular architecture.

Comment
Hydralazine, or 1-hydrazinylphthalazine, is a direct-acting smooth muscle relaxant used to treat hypertension by acting as a vasodilator, primarily in arteries and arterioles. Upon condensing with carbonyl compounds hydralazine will form hydrazones, namely 1-phthalazinyl hydrazones, which find use as vasodilating antihypertensive drugs and also application in optoelectronics (Caruso et al., 2005).
The title compound is one such 1-phthalazinyl hydrazone. It crystallizes in the triclinic, P1, space group. The molecule exists in its E configuration with respect to the C7=N1 bond which is confirmed by the torsion angle of 177.11 (12)° of the C6-C7-N1-N2 moiety (Fig. 1). The torsion angle of -5.33 (17) Shafiq et al., 2013), confirming the azomethine bond formation and the presence of a hydrazinylidene. The phenol, azomethine and phthalazin moieties are nearly planar (rms deviations 0.0041, 0.0000 and 0.0328 Å respectively) and coplanar to each other, with the two moieties at the ends of the molecule slightly twisted away from the central moiety in opposite directions by torsion angles of 7.67 (10) and 8.68 (11)° for the phenol and phthalazin moieties with the central azomethine moiety, respectively. The dihedral angle between phenol and phthalazin moieties is 15.98 (7)°.
These layers are in turn connected with neighboring layers through the intermolecular N-H···N and C-H···O H-bonds ( Fig. 6) to yield a supramolecular architecture sustained by H-bond interactions and π-π interactions. Fig. 7 shows the packing of the molecules along the a axis.

Refinement
All H atoms on C were placed in calculated positions, guided by difference maps, with C-H bond distances of 0.93 Å. H atoms were assigned U iso (H) values of 1.2Ueq(carrier). The phenolic O-H distance was restrained to 0.84 (2) Å. The phenolic H atom was found to be disorderd by tautomerism over two positions: partially bonded to O1 and partially bonded to N1 (where the largest Q peak is located after inclusion of extinction correction) with refined occupancies of 0.80 (3) and 0.20 (3) respectively. Partial occupancy of H1 at O1 was also indicated by a rather large U iso value for H1A of 0.103 before inclusion of disorder. The U iso value for H1B was set to 1.2 times of U eq of the N1 atom. H3′, located from a difference map, was refined with an N-H distance restraint of 0.88 (2) Å and has a refined U iso value of 0.058 Å 2 .

Figure 1
ORTEP view of the compound, drawn with 50% probability displacement ellipsoids for the non-H atoms (the minor moiety H atom was omitted for clarity).      Graphical representation showing π-π interactions between phenol and pthalazin rings in the crystal structure of the title compound.

Figure 5
Graphical representation showing π-π interactions that lead to formation of infinite layers in the crystal structure of the title compound.  Graphical representation showing neighboring layers formed by π-π interactions and connected through intermolecular

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