Synthesis, molecular and crystal structure of 1-(1,2-dihydrophthalazin-1-ylidene)-2-[1-(thiophen-2-yl)ethylidene]hydrazine

The two independent molecules in the asymmetric unit of the title compound are connected via two N—H⋯N hydrogen bonds, forming dimers which interact by two bifurcated π–π stacking interactions to build tetrameric motifs. These are packed via C—H ⋯N and C—H⋯π interactions, resulting in a three-dimensional architecture with a tilted herringbone packing mode.

The title compound, C 14 H 12 N 4 S, was synthesized by the condensation reaction of hydralazine and 2-acetylthiophene and during the reaction, a proton transfer from the imino nitrogen atom to one of the endocylic nitrogen atoms occurred. The compound crystallizes in the monoclinic crystal system with two independent molecules (molecules 1 and 2) in the asymmetric unit. In each molecule, there is a slight difference in the orientation of the thiophene ring with respect to phthalazine ring system, molecule 1 showing a dihedral angle of 42.51 (1) compared to 8.48 (1) in molecule 2. This implies an r.m.s deviation of 0.428 (1) Å between the two molecules for the 19 non-H atoms. The two independent molecules are connected via two N-HÁ Á ÁN hydrogen bonds, forming dimers which interact by two bifurcatedstacking interactions to build tetrameric motifs. The latter are packed in the ac plane via weak C-HÁ Á Á interactions and along the b axis via C-H Á Á ÁN and C-HÁ Á Á interactions. This results a three-dimensional architecture with a tilted herringbone packing mode.

Chemical context
Hydralazine compounds are being studied intensively for their biological and chemical properties, the former giving them interesting pharmacological properties (antimicrobial, antimalarial and antitumor activity; Jackson et al., 1990;Kaminskas et al., 2004;Vicini et al., 2006). They also find wide applications in the treatment of tuberculosis, leprosy and mental disorder. Furthermore, there is considerable research interest in 1-hydrazinophthalazine (hydralazine) because its hydrochloride is an effective drug for the emergency reduction of blood pressure in hypertensive crises (Draey & Tripod, 1967). It has also been reported that a combination of hydralazine and hydrochlorothiazide is being used to treat high blood pressure, as they work by relaxing blood vessels and increasing the supply of blood oxygen to the heart while reducing its workload (Shoukry & Shoukry, 2008). The chemical properties of hydrazone compounds are also interesting because their nature as polydentate ligands makes them very versatile molecules. The physiological importance of hydralazine derivatives has led to great interest in their complexation tendency with metal ions, especially with transition-metal ions of biological importance. The coordination chemistry of hydrazones is being studied in connection with their increasing use as pharmaceuticals and analytical reagents. Few complexes of 1-phthalazinylhydrazone have been reported (Al'-Assar et al., 1992; Kogan et al., 2009;Holló ISSN 2056-9890 et al., 2014Bakale et al., 2014b;Levchenkov et al., 2015). In a continuation of our studies of hydralazine derivatives and their complexes (Nfor et al., 2013;Majoumo-Mbe et al., 2015), we herein report the preparation and the structural study of the title compound, also known as 2-acetylthiophene-1phthalazinylhydrazone.

Structural commentary
The title compound crystallizes in the monoclinic crystal system (space group P2 1 /n) with two independent molecules, 1 and 2, in the asymmetric unit, as shown in Fig. 1 (atoms in molecule 2 have the suffix B).
There are slight differences between the molecules, as shown in Fig. 2, with an r.m.s. fit of 0.428 (1) Å for the 19 non-H atoms. This deviation arises from the different orientations of the thiophene moiety. The dihedral angle between the thiophene ring and the phthalazine ring system is 42.51 (1) in molecule 1 compared to 8.48 (1) in molecule 2.
In both molecules, the thiophene rings (C10-C13/S1) are in a planar conformation with a maximum deviation of 0.006 (1) Å for S1 in molecule 1 and of 0.003 (1) Å for S1B in molecule 2. The phthalazine ring systems are also essentially planar with maximum deviations from the best plane of the ten-membered ring systems of 0.003 (1) Å for N1 in molecule 1 and 0.022 (1) Å for C8B in molecule 2. The lengths of the N4-C9 and N4B-C9B bonds of 1.294 (2) and 1.296 (2) Å , respectively, are in agreement with that of an N Csp 2 bond (1.282 AE0.060) Å found in the CSD (Version 5.39, update of August 2018; Groom et al., 2016) for acyclic nitrogen and carbon atoms in organic compounds. This confirms the condensation reaction between the two reagents. The hydrogen atoms H2 and H2B bonded respectively to N2 and N2B (see Table 1) indicate that proton transfer from the imino nitrogen atoms N3 and N3B has occurred. The latter is confirmed by the double-bond character of N3-C8 [1.306 (2) Å ] and N3B-C8B [1.309 (2) Å ] and the singlebond character of N3-N4 [1.398 (2) Å ] and N3B-N4B [1.400 (2) Å ]. Indeed, these values are in agreement with the bond lengths for C N and N-N bonds (1.3 AE 0.1 and 1.4 AE 0.1 Å , respectively) in the C N-N fragment with a cyclic carbon atom and acyclic nitrogen atoms for organic compounds in the CSD. Such a proton transfer has been reported in other hydrazinophthalazine derivatives (Ianelli et al., 2002;Butcher et al., 2007;Popov et al., 2012;Nfor et al., 2013;Majoumo-Mbe et al., 2015).

Supramolecular features
Molecules 1 and 2 are linked via two N-HÁ Á ÁN hydrogen bonds (see Table 1), forming dimers which are held together by two bifurcatedinteractions (Table 1)  A view of the overlay (Mercury; Macrae et al., 2006) of the two independent molecules (colour code: red = molecule 1, black = molecule 2).

Figure 1
Molecular structure of the two independent molecules (1 and 2) with the labelling scheme. Displacement ellipsoids are drawn at the 50% probability level. H atoms have been omitted for clarity. thiazol-2-yl)thiophene (Nguyen Ngoc et al., 2017). The resulting tetramers in the title compound are packed in a tilted herringbone motif. As shown in Fig. 4, they interact via the C13-H13Á Á ÁN1 i hydrogen bonds and C3B-H3BÁ Á ÁCg4 ii interactions along the b-axis direction ( Fig. 4) and in the ac plane via C11-H11Á Á ÁCg2 iii interactions (Fig. 5). The resulting packing shows small voids of 12.94 Å 3 (0.5% of the unit cell; calculated with a probe radius of 1.2 Å by using the contact surface).  (Bakale et al., 2014a). None of these crystals exhibits a packing mode with a tetrameric motif similar to that reported in this work.

Synthesis and crystallization
The title molecule was prepared by condensation of 2-acetylthiophene (2.54 g, 20 mmol) and hydralazine hydro-chloride (3.94 g, 20 mmol) in 20 ml of methanol and 10 ml of aqueous solution of sodium acetate (1.64 g, 20 mmol) as buffering agent. The mixture was refluxed at 338 K under stirring for four h. The product was left overnight to cool. The yellow precipitate was filtered off and washed several times with water and methanol, and finally crystallized from a mixture of DMF/methanol (2:1) as yellow crystals (in a yield of around 80%) suitable for single-crystal X-ray diffraction studies. Table 1 Hydrogen-bond geometry (Å , ).

Figure 3
The packing of dimers of molecules 1 and 2 (symmetry code as in Table 1).