A second polymorph of (Z)-3-amino-4-(2-phenylhydrazinylidene)-1H-pyrazol-5(4H)-one

The molecule of the title compound, C9H9N5O, is approximately planar (the r.m.s. deviation of all non-H atoms is 0.08 Å). The amine substituent is pyramidal at the N atom. An intramolecular N—Hhydrazine⋯O=C hydrogen bond is present. In the crystal, molecules are connected via N—H⋯N and N—H⋯O hydrogen bonds, forming infinite layers parallel to (010). This polymorph is triclinic, space group P-1, whereas the previously reported form was monoclinic, space group P21/c [Elgemeie et al. (2013 ▶). Acta Cryst. E69, o187], with stepped layers and a significantly lower density.

The molecule of the title compound, C 9 H 9 N 5 O, is approximately planar (the r.m.s. deviation of all non-H atoms is 0.08 Å ). The amine substituent is pyramidal at the N atom. An intramolecular N-H hydrazine Á Á ÁO C hydrogen bond is present. In the crystal, molecules are connected via N-HÁ Á ÁN and N-HÁ Á ÁO hydrogen bonds, forming infinite layers parallel to (010). This polymorph is triclinic, space group P1, whereas the previously reported form was monoclinic, space group P2 1 /c [Elgemeie et al. (2013). Acta Cryst. E69, o187], with stepped layers and a significantly lower density.
The molecule of It is shown in Fig. 1. Molecular dimensions, such as the bond lengths C4═N4 1.3070 (10) and N4-N5 1.3128 (9) Å, may be regarded as normal. A search of the Cambridge Database (Allen, 2002; Version 1.15) for the same five-membered ring with C═O and C═N-N substituents gave average bond lengths of 1.309 (8) and 1.318 (10) Å respectively for these bonds (35 hits, 39 fragments excluding one obvious outlier). The nitrogen N3 of the amine substituent is pyramidal, lying 0.224 (8) Å out of the plane of its substituents. The entire molecule is planar to within an r.m.s. deviation of 0.08 Å for the non-H atoms, with no torsion angle deviating by more than 6.5 ° from 0/180 °; this is in part attributable to the intramolecular hydrogen bond N5-H05···O1. A more detailed analysis shows two planar units (i) C11-16 with substituent N5 and (ii) the five-membered ring, both with r.m.s.d. 0.004 Å, that subtend an interplanar angle of 8.51 (4) °. The substituents O1, N4 and N3 all lie slightly but significantly out of the plane of the five-membered ring, by 0.057 (1), 0.095 (1) and 0.069 (1) Å respectively, all to the same side. A comparison with Im shows that the molecules are closely similar except for a slight difference in the orientation of the phenyl ring; a least-squares fit excluding the nonipso phenyl carbon atoms gives an r.m.s. deviation of 0.04 Å. However, there are some significant differences such as the N1-C5 bond length, which is 1.3387 (13) Å in Im but 1.3489 (10) Å in It.
The molecules of It are connected by hydrogen bonds (Table 1) to form infinite layers (one per b translation) parallel to the ac plane (Fig. 2). There are two three-centre interactions (N5-H05···O1 intra-and intermolecular, N1-H01···N1/N2) and two two-centre interactions involving the NH 2 H atoms (with N2 and O1 as acceptors). In Im, one of the N3 H atoms forms a three-centre system with N1 and N2, the N5-H05···O1 interaction is purely two-centre, N1-H01 and N3-H03A are two-centre donors to O1. The packing of Im is a stepped layer structure that intuitively seems to display a less efficient packing than that of It; consistent with this are the crystallographic densities of 1.464 and 1.491 g cm -3 , respectively, but we have carried out no further calculations or experiments to test this hypothesis.

Experimental
The title compound was obtained according to the following general procedure (Arnost et al., 2010): 3-amino-1Hpyrazol-5(4H)-one was prepared by refluxing an ethanolic solution of sodium cyanoacetate and sodium ethoxide containing a few drops of hydrazine hydrate for 1 h. After cooling, the precipitate was filtered off and recrystallized from ethanol. A solution of freshly prepared phenyldiazonium chloride (2.66 mmol) was added to the pyrazolone (1 equiv in 50% aqueous EtOH, 3 ml/mmol)and potassium acetate (6 equiv) and the mixture stirred at 0 °C for a further 30 min. The precipitate was filtered, washed with water and dried to give (

Refinement
The NH H atoms were refined freely. Other H 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 for these H.

Figure 1
The molecule of the title compound in the crystal; ellipsoids represent 50% probability levels. Packing diagram of the title compound viewed perpendicular to the ac plane. Phenyl rings are reduced to the ipso carbons for clarity. Intramolecular hydrogen bonds are drawn as thin and intermolecular hydrogen bonds as thick dashed lines; the latter are numbered according to their order in the relevant Table. H bonds #4 are omitted because they represent the weak components of asymmetric three-centre interactions.  Rms deviation of fitted atoms = 0.0000 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.