N,N′-Dihydroxybenzene-1,2:4,5-tetracarboximide dihydrate

In the title compound, C10H4N2O6·2H2O, the organic molecule has crystallographically imposed inversion symmetry. The atoms of the three fused rings of the molecule are coplanar within 0.0246 (8) Å, while the two hydroxy O atoms are displaced from the mean plane of the molecule by 0.127 (1) Å. In the crystal, infinite near-planar layers of close-packed molecules are formed by hydrogen bonding between water O—H donor groups and carbonyl O-atom acceptors, and by weak interactions between C—H donor groups and water O-atom acceptors. The layers are parallel to the {102} family of planes. The stacked planes are held together by hydrogen bonding between N—OH donor groups and water O-atom acceptors.

In the title compound, C 10 H 4 N 2 O 6 Á2H 2 O, the organic molecule has crystallographically imposed inversion symmetry. The atoms of the three fused rings of the molecule are coplanar within 0.0246 (8) Å , while the two hydroxy O atoms are displaced from the mean plane of the molecule by 0.127 (1) Å . In the crystal, infinite near-planar layers of close-packed molecules are formed by hydrogen bonding between water O-H donor groups and carbonyl O-atom acceptors, and by weak interactions between C-H donor groups and water Oatom acceptors. The layers are parallel to the {102} family of planes. The stacked planes are held together by hydrogen bonding between N-OH donor groups and water O-atom acceptors.
Data collection: COLLECT (Nonius, 1999); cell refinement: DIRAX/LSQ (Duisenberg et al., 2000); data reduction: EVALCCD (Duisenberg et al., 2003); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and Mercury (Macrae et al., 2006); software used to prepare material for publication: WinGX (Farrugia, 2012 The crystal engineering of structures containing stacked infinite planar layers of H bonded aromatic molecules is a hot research topic because of the potential interest of such structures as advanced materials in organic electronics, optoelectronics and photonics (Centore, Causà et al. 2013). Conjugated heterocyclic aromatic compounds are often used as building blocks for assembling active molecules for those advanced applications (Carella et al., 2004;Centore, Concilio et al., 2012). Aromatic diimides, in particular, are well known for their outstanding properties as n-type organic semiconductors (Centore, Ricciotti et al., 2012). Following these issues and our basic interest for crystal structures of conjugated heteroaromatic compounds conditioned by the formation of strong and weak H bonds (Centore et al., 2013a; 2013b) we report the structural investigation of the title compound, which is the dihydrate form of the N,N′-dihydroxy derivative of the simplest aromatic bis(imide).
The molecular structure is shown in Fig. 1 Fig. 2(b), adjacent stacked layers are related by a 2 1 screw operation, which is a very efficient way to fulfill the Kitaigorodskii's "bumps in hollows" golden rule of the close packing of layers (Kitaigorodskii, 1961).

Experimental
Hydroxylamine hydrochloride (3.18 g, 45.8 mmol) was added to pyridine (25 ml) and stirred at room temperature for 10 min until a clear solution was obtained. Pyromellitic anhydride (5.00 g, 22.9 mmol) was added and the solution was refluxed overnight. The solution was cooled to room temperature and the solid precipitate was filtered and washed with methanol and dried in oven at 120 °C for three days. Then it was poured in 100 ml of a solution methanol/conc. HCl Single crystals for X-ray analysis were obtained by slow evaporation of an ethanol/water (1:1 v/v) solution.

Refinement
The H atoms bonded to O atoms were located in a difference Fourier map and their coordinates were refined. The H atom on benzene ring was generated stereochemically and was refined by the riding model. For all H atoms U iso (H) = 1.2U eq (C, O) was assumed.

Computing details
Data collection: COLLECT (Nonius, 1999); cell refinement: DIRAX/LSQ (Duisenberg et al., 2000); data reduction: EVALCCD (Duisenberg et al., 2003); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and Mercury (Macrae et al., 2006); software used to prepare material for publication: WinGX (Farrugia, 2012).  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.