4,6-Dinitropyrogallol

In the title molecule, C6H4N2O7, the two nitro groups are tilted with respect to the aromatic ring by 11.2 (1) and 10.9 (1)°. All three hydroxy groups are involved in the formation of bifurcated intra- and intermolecular O—H⋯O hydrogen bonds. The crystal packing exhibits short O⋯O distances of 2.823 (2) Å between two O atoms of the nitro groups.

In the title molecule, C 6 H 4 N 2 O 7 , the two nitro groups are tilted with respect to the aromatic ring by 11.2 (1) and 10.9 (1) . All three hydroxy groups are involved in the formation of bifurcated intra-and intermolecular O-HÁ Á ÁO hydrogen bonds. The crystal packing exhibits short OÁ Á ÁO distances of 2.823 (2) Å between two O atoms of the nitro groups.

Experimental
Data collection: IPDS Software (Stoe & Cie, 1997); cell refinement: IPDS Software; data reduction: IPDS Software; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2011); software used to prepare material for publication: SHELXL97. As expected, the aromatic C 6 -ring is planar (the positions of the six C atoms deviate from the mean plane by 0.0025 (8)-0.0196 (8) Å). The C 6 -ring and the two nitro groups are also approximately coplanar, allowing significant πdelocalization. In the extended hydrogen bonding network all three phenolic hydroxy groups act as hydrogen donors and the nitro groups as hydrogen acceptors. The O atoms O1 and O6 of two neighbouring molecules form a planar, fourmembered O1···O6···O1′···O6′ ring with two bifurcated hydrogen bonds. Remarkably, the O6···O6′ distance of 2.823 (2) Å (i.e. one of the diagonal of the corresponding parallelogram) is somewhat shorter than the sum of the van der Waals radii, although no direct hydrogen bond between these two atoms is operative. Some intermolecular C···C and C···O distances (C1···C1′: 3.279 Å, C5···O2": 3.046 Å) are slightly shorter than the sum of the van der Waals radii and may be interpreted in terms of π-stacking or weak donor-acceptor interactions.

Experimental
The title compound has been prepared by nitration of pyrogallol-triacetate. 1

Refinement
All non-hydrogen atoms were refined using anisotropic displacement parameters. H atoms were treated as recommended by Müller et al. (2006). A riding model was used for the C-bonded H5. The positional parameters of the O-bonded H1O, H2O and H3O were refined using isotropic displacement parameters which were set to 1.5U eq of the pivot atom. In addition, a restraint of 0.84 Å was used for the O-H distances.

Figure 2
Section of the hydrogen bonding network, showing bifurcated hydrogen bonds for all three hydroxy groups (displacement ellipsoids are drawn at the 50% probability level). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.31 e Å −3 Δρ min = −0.23 e Å −3 Extinction correction: SHELXL97 (Sheldrick, 2008), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.093 (9) 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq C5 0.03683 (