(E)-3-Amino-4-(2-phenylhydrazinylidene)-1H-pyrazol-5(4H)-one

The molecule of the title compound, C9H9N5O, is essentially planar (r.m.s. deviation of all atoms = 0.02 Å) except for the NH2 H atoms. An intramolecular hydrazinylidene–carbonyl N—H⋯O=C hydrogen bond is present. In the crystal, molecules are connected via N—H⋯N/O hydrogen bonds, forming thick layers parallel to (100).


Experimental
Crystal data H atoms treated by a mixture of independent and constrained refinement Á max = 0.17 e Å À3 Á min = À0.26 e Å À3 Table 1 Hydrogen-bond geometry (Å , ).  Chemically synthesized purine analogues find numerous applications in clinical medicine and medical research (Elgemeie, 2003;Elgemeie et al., 2008). The pharmacological approach involves analogues in which the heterocyclic ring system has been modified so as to induce toxic effects when the analogue is incorporated into specific cell constituents (Elgemeie & El-Aziz, 2002). As part of our program directed towards the synthesis of purines and other antimetabolites (Elgemeie et al., 2001(Elgemeie et al., , 2009, we have recently reported various successful approaches to the syntheses of purine analogues. Derivatives of these ring systems are of interest as antimetabolites in biochemical reactions (Elgemeie & Sood, 2003). We have described several novel syntheses of functionalized pyrazoles (Elgemeie et al., 2007).
These compounds are considered important intermediates for the synthesis of various purine ring systems (Elgemeie & Sood, 2006). As a continuation of this work, the title pyrazole compound (2), was prepared as a precursor for the synthesis of other purines. 2-Hydrazinyl-2-oxo-N-phenylacetohydrazonoyl cyanide (1) undergoes intramolecular cyclization by refluxing in ethanol containing catalytic amounts of piperidine to give the novel pyrazole derivative (2).
The title compound can potentially exist in two other tautomeric forms with hydroxyl groups, (3) and (4). Spectral studies, however, indicated the presence of the ketonic tautomer (2) in solution (e.g. the 13 C NMR signal at δ = 174.00, indicating a carbonyl carbon rather than C-OH. The X-ray analysis of (2) ( Fig. 1) establishes the exclusive presence of the keto tautomer in the solid state; all H atoms could be located unambiguously and bond lengths are also consistent with the keto form. The entire molecule is planar (r.m.s. deviation of all non-C atoms: 0.02 Å), except for the H atoms of the NH 2 group; H03A lies 0.36 (2) and H03B 0.27 (2) Å outside the plane. Consistent with the E configuration, an intramolecular hydrogen bond N5-H05···O1 is observed.
The molecules are connected by hydrogen bonds #1-#3 to form thick hydrogen-bonded layers parallel to (100); the individual molecules are to a good approximation oriented in the planes (042) (Figs. 2, 3). Hydrogen bond #4 is the second and appreciably less linear branch of a three-centre interaction.

Experimental
The title compound was obtained by refluxing an ethanolic solution of 2-hydrazinyl-2-oxo-N-phenylacetohydrazonoyl cyanide containing a few drops of piperidine for 1 h. After cooling, the precipitate was filtered off and recrystallized from

Refinement
The NH H atoms were refined freely. Other H atoms were placed in calculated positions and refined using a riding model with C-H arom 0.95 Å; the hydrogen U values were fixed at 1.2 × U(eq) of the parent atom.

Figure 1
Molecular structure of the title compound. Ellipsoids represent 50% probability levels.  Packing diagram of the title compound, viewed perpendicular to (100). Thick dashed bonds represent classical H bonds.
Atom names correspond to the asymmetric unit; hydrogen bonds are numbered according to the Table on page Sup-7 (#4, the weaker part of a three-centre interaction, is omitted, as is the intramolecular interaction #5).

Figure 4
The formation of the title compound Rms deviation of fitted atoms = 0.0175 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.