( E , E )-1-( 3-Iodophenyl )-4-( 3-nitrophenyl )-2 , 3-diazabuta-1 , 3-diene

Department of Chemistry, University of Aberdeen, Meston Walk, Old Aberdeen AB24 3UE, Scotland, Department of Chemistry, University of Aberdeen, Meston Walk, Old Aberdeen AB24 3UE, Scotland, Instituto de Quı́mica, Departamento de Quı́mica Inorgânica, Universidade Federal do Rio de Janeiro, 21945-970 Rio de Janeiro, RJ, Brazil, and School of Chemistry, University of St Andrews, Fife KY16 9ST, Scotland

Molecules of the title compound, C 14 H 10 INO 2 , apparently containing a small proportion of the compound with a second nitro group replacing iodo, are disordered across centres of inversion. The molecules are linked into ordered chains by a two-centre iodo-nitro interaction and these chains are linked into sheets by C-HÁ Á ÁO hydrogen bonds.
The central -CH N-N CH-unit is strictly planar in (I) and the substituents at each of the C N bonds adopt E configurations; the aryl groups are only slightly twisted away from the plane of the central spacer, as shown by the relevant torsion angle (Table 1) and, similarly, the nitro group is almost coplanar with the adjacent aryl ring. The occupancy of the nitro group was found to be greater than that of the iodo substituent with site-occupancy factors of 0.587 (4) and 0.413 (4), respectively. A similar phenomenon was found in the isomeric compound (E,E)-1-(2-iodophenyl)-4-(2-nitro-phenyl)-2,3-diazabuta-1,3-diene, (II), where the molecules are disordered across inversion centres in the space group C2/c. In (II), however, the population of the nitro sites was smaller than that of the iodo site. As in (II), we conclude that some reorganization of the substituted aryl groups has occurred in (I), either during the synthesis or the crystallization, and that a small proportion of (E,E)-1,4-bis(3-nitrophenyl)-2,3-diazabuta-1,3-diene, (III), has cocrystallized with (I).
The angular properties of the C-IÁ Á ÁI interaction here are consistent with generalizations proposed (Ramasubbu et al., 1986) from the results of database analysis, namely that in structures where XÁ Á ÁX distances (X = halogen) are significantly less than the van der Waals sum, the observed C-XÁ Á ÁX angles are clustered around either 180 or 90 . The short IÁ Á ÁI distance found in (I) is well below the conventional van der Waals sum for an IÁ Á ÁI contact (3.90 Å ; Bondi, 1964) and still well below the revised value (3.52 Å ) established from the polar-flattening model (Nyburg & Faerman, 1985). This may point to an avoidance of such contacts in (I) wherever possible; such contacts are readily avoided if the molecules within each [010] chain are aligned in a head-to-tail fashion, so that the disorder of the molecules is correlated within each [010] chain; however, this correlation neither requires nor implies any correlation of disorder between adjacent chains. Thus we conclude that the molecules of (I) are linked into [010] chains by a two-centre iodo-nitro interaction, and that short IÁ Á ÁI contacts are, in fact, absent from this structure.
There are two possible C-HÁ Á ÁO hydrogen bonds within the structure (Table 2), whose structural influence is intimately bound up with the orientational disorder of the molecules. Atoms C5 and C6 at (x, y, z), which are components of the molecule centred at ( 1 2 , 1 2 , 1 2 ), itself part of the chain along ( 1 2 , y, 1 2 ), act as hydrogen-bond donors, respectively, to atoms O32 at (1 + x, 3 2 À y, 1 2 + z) and O31 at (2 À x, À 1 2 + y, 3 2 À z), which themselves are components of the molecules centred across ( 3 2 , 1, 1) and ( 3 2 , 0, 1), respectively, which are components of the chain lying along ( 3 2 , y, 1). A given aryl ring can act only as a single donor of hydrogen bonds, and which of these is actually formed by this pair of C-H bonds in a given aryl ring depends only upon the relative orientation of the [010] chains containing the potential donors and acceptors. If the chains along ( 1 2 , y, 1 2 ) and ( 3 2 , y, 1) are aligned in a parallel fashion, they will be linked by the hydrogen bond formed by C6, but if they are aligned antiparallel they will be linked by the hydrogen bond formed by C5. However, there will always be exactly one such interaction present for each aryl ring. Regardless of the local connectivity, the overall effect of the hydrogen bonds is to link [010] chains into a (102) sheet.
By contrast with the occurrence of C-HÁ Á ÁO hydrogen bonds in compound (I), no such bonds occur in the isomer (II). We note without comment that in both of these disordered structures, (I) and (II), the unit cells are of markedly tabular shape, with a short a dimension in (I) and a short b dimension of 3.7952 (3) Å in (II).

Experimental
An equimolar mixture of 3-iodobenzaldehyde and 3-nitrobenzaldehyde hydrazone (3 mmol of each) in methanol (20 ml) was heated under reflux for 20 min; the mixture was cooled and the resulting solid product, (I), was collected by filtration. Crystals suitable for single-crystal X-ray diffraction were grown by slow evaporation of a solution in 1,2-dichloroethane.  The molecule of (I), showing the atom-labelling scheme. Atoms marked with an 'a' are at the symmetry position (1 À x, 1 À y, 1 À z). Displacement ellipsoids are drawn at the 30% probability level. Table 1 Selected torsion angles ( ).
All H atoms were located in a difference Fourier map and then treated as riding atoms, with C-H distances of 0.95 Å and U iso (H) = 1.2U eq (C). The refined values of the site-occupancy factors for the nitro group and the I atom were 0.587 (4) and 0.413 (4), respectively.