4-Nitrophenol–piperazine (2/1)

In the title adduct, C6H5NO3·0.5C4H10N2, the piperazine ring possesses inversion symmetry and has a chair conformation. Its mean plane makes a dihedral angle of 65.45 (7)° with the 4-nitrophenol ring. In the crystal, the piperazine ring is linked to two 4-nitrophenol molecules via O—H⋯N hydrogen bonds. The molecules are also linked via bifurcated N—H⋯(O,O) hydrogen bonds involving the NO2 O atoms, forming a two-dimensional network lying parallel to (102). The networks are linked via C—H⋯O hydrogen bonds, forming a three-dimensional structure.

In the crystal, the piperazine ring is linked to two 4-nitrophenol molecules via O-H···N hydrogen bonds (Table 1 and Fig   2). The molecules are also linked via bifurcated N-H···O/O hydrogen bonds, involving the NO 2 O atoms, forming a twodimensional network lying parallel to (102). These networks are linked via C-H···O hydrogen bonds forming a threedimensional structure (Table 1).

Experimental
Piperazine 4-nitrophenol was synthesized by mixing an equimolar mixture (1:1) of anhydrous piperazine and 4-nitrophenol in methanol. The resultant solution was stirred magnetically at room temperature and filtered into a clean beaker.
The filtrate was kept in a constant temperature bath at 308 K. Yellow block-like crystals suitable for x-ray diffraction were harvested from the solution within a day.

Refinement
The OH and C-bound H atoms were positioned geometrically and refined using a riding model: O-H = 0.82 Å, C-H = 0.93 and 0.97 Å for aryl and methylene H-atoms, respectively, with U iso (H) = 1.5U eq (O) and = 1.2U eq (C,N).

Figure 2
The crystal packing of the title compound viewed along the a axis. The N-H···O, O-H···N and C-H···O hydrogen bonds are shown as dashed lines; see Table 1

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 > 2sigma(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.