4-Nitroanilinium triiodide monohydrate

In the title compound, C6H7N2O2 +·I3 −·H2O, the triiodide anions form two-dimensional sheets along the a and c axes. These sheets are separated by the 4-nitroanilinium cations and water molecules, which form part of an extended hydrogen-bonded chain with the triiodide along the c axis, represented by the graph set C 3 3(14). The second important hydrogen-bonding interaction is between the nitro group, the water molecule and the anilinium group, which forms an R 2 2(6) ring and may be the reason for the deviation of the torsion angle between the benzene ring and the nitro group from 180 to 163.2 (4)°. These two strong hydrogen-bonding interactions also cause the benzene rings to pack off-centre from one another, with an edge-on-edge π–π stacking distance of 3.634 (6) Å and a centroid–centroid separation of 4.843 (2) Å.

In the title compound, C 6 H 7 N 2 O 2 + ÁI 3 À ÁH 2 O, the triiodide anions form two-dimensional sheets along the a and c axes. These sheets are separated by the 4-nitroanilinium cations and water molecules, which form part of an extended hydrogenbonded chain with the triiodide along the c axis, represented by the graph set C 3 3 (14). The second important hydrogenbonding interaction is between the nitro group, the water molecule and the anilinium group, which forms an R 2 2 (6) ring and may be the reason for the deviation of the torsion angle between the benzene ring and the nitro group from 180 to 163.2 (4) . These two strong hydrogen-bonding interactions also cause the benzene rings to pack off-centre from one another, with an edge-on-edgestacking distance of 3.634 (6) Å and a centroid-centroid separation of 4.843 (2) Å .

Comment
Previously 4-nitroanilinine was crystallized with bromine (Lemmerer & Billing, 2006) and chlorine (Ploug-Sørensen et al., 1982) to produce the respective monohalide salts. In an attempt to synthesize a monoiodide salt with 4-nitroaniline, the black crystals of 4-nitroanilinium triiodide monohydrate, C 6 H 7 N 2 O 2 + I 3 -. H 2 O (I) formed in preference, and the structure is reported here. Polyiodide salts are commonly found, but the triiodides less so. Tebbe & Loukili (1998)  In the structure of (I) (Figs. 1, 2), the triiodide anions essentially form two-dimensional sheets along the a and c axes.
Looking at the interactions along the a axis, the layers of triiodide anions pack parallel to each other with a separation of 4.843 (1) Å. The two intermolecular head-to-tail I1···I1 and the two I3···I3 interactions along the c axis have a separation of 4.574 (1) and 3.772 (1) Å and 4.1079 (7) and 5.2776 (8) Å respectively, completing the interactions which form the two-dimensional sheets. These sheets are separated by the 4-nitroanilium and water moieties which form part of an extended hydrogen-bonded chain with the triiodide along the c axis of the unit cell, represented by the graph set C 3 3 (14) (Etter et al., 1990). The graph set notation includes H···I hydrogen bonds with the water and the nitro oxygen (O2) i.e. (O3-H···O2), as seen in Fig 2. Besides the strong C 3 3 (14) hydrogen-bonding network, another important hydrogen-bonding association is between the nitro group, the water and the ammonium group, forming an R 2 2 (6) ring (Table 1). This ring appears to be an important interaction which gives a deviation of the torsion angle C6-C1-N1-O2 between the benzene ring and the nitro group from 180° to 163.2 (4)°. The two strong hydrogen-bonding interactions result in the benzene rings packing off-centre from one another with an edge-on-edge π-π stacking distance of 3.634 (6) Å and a centroid-to-centroid separation of 4.843 (2) Å.
The many short intermolecular distances between the triiodide anions and the benzene rings may be important in the optical properties of (I), regarding charge-transfer interactions and conductivity, as found in this type of compound (Shibaeva & Yagubskii, 2004).

Experimental
For the preparation of (I) 0.632 g of 4-nitroaniline was dissolved in 4 ml of 55% aqueous HI. The solution was heated to dissolve the precipitate and then left to stand at room temperature. Crystals suitable for single crystal X-ray diffraction were grown by slow evaporation of the solvent over a period of one month.

supplementary materials sup-2 Refinement
The H atoms on nitroaniline were refined using a riding-model, with C-H = 0.93 Å, N-H = 0.89 Å and with U ĩso (H) = 1.2U eq (C) or 1.5U eq (N). The H atoms on the water were placed from the difference Fourier map with O-H = 0.90 (2) Å and constrained using the DFIX constraint (Sheldrick, 2008). The highest residual electron density peak (0.708eÅ -3 ) was 0.865 Å from I2.

Special details
Experimental. Numerical integration absorption corrections based on indexed crystal faces were applied using the XPREP routine (Bruker, 1999a) 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.