3,3′-Diazenediyldiphthalic acid dihydrate

In the crystal structure of the title compound, C16H10N2O8·2H2O, the organic molecule is located on a centre of symmetry. The two benzene rings are parallel, but not coplanar, as indicated by N=N—C—C torsion angles involving the azo group of 12.1 (5) and −168.2 (3)°. The organic molecule and the water molecule are linked by O—H⋯O hydrogen bonds, forming a three-dimensional network.

In the crystal structure of the title compound, C 16 H 10 N 2 O 8 Á2H 2 O, the organic molecule is located on a centre of symmetry. The two benzene rings are parallel, but not coplanar, as indicated by N N-C-C torsion angles involving the azo group of 12.1 (5) and À168.2 (3) . The organic molecule and the water molecule are linked by O-HÁ Á ÁO hydrogen bonds, forming a three-dimensional network.

Comment
In an attempt to prepare a Cd-containing coordination polmyer (Carlucci et al., 2000), the title compound was obtained as an unexpected product.
The complete organic molecule ( Fig. 1) is generated by inversion at the midpoint of the central N-N bond and a water molecule of crystallization completes the crystal structure. The components interact through O-H···O hydrogen bonds (Table 1) to generate a three-dimensional architecture.

Experimental
CdSO 4 (0.033 g, 0.012 mmol), 2,2',3,3'-diazenediyldiphthalic acid (0.026 g, 0.014 mmol) and NaOH (0.048 mmol, 0.12 mmol), were added in a mixed solvent of acetonitrile and the mixture was heated for ten hours under reflux. During the process stirring and influx were required. The resultant was then filtered to give a pure solution which was infiltrated by diethyl ether freely in a closed vessel. Single crystals suitable for X-ray diffraction were obtained for a week.

Refinement
C-bound H atoms were placed at calculated positions (C-H = 0.93 Å) and were treated as riding, with U iso (H) = 1.2U eq (C).
Carboxy H atoms were constrained with O-H = 0.82 Å and U iso (H) = 1.2U eq (O), but each O-H group was allowed to rotate freely about its C-O bond. Water H atoms were tentatively located in a difference Fourier map and were refined, with distance restraints of O-H = 0.85 (1) Å and H···H = 1.39 (1) Å, and with U iso (H) = 1.2U eq (O). The maximum residual peak is located 1.34 Å from Br1. Fig. 1  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 calculat-supplementary materials sup-3 ing 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.