Butane-2,3-dione bis[(4-bromobenzylidene)hydrazone]

The title compound, C18H16Br2N4, is a linear double Schiff base compound having two parallel 4-bromophenyl groups connected across a crystallographic inversion centre by flexible C—C and C=N—N=C bonds and stabilized in the solid state by weak intermolecular Br⋯Br interactions [3.7992 (11) Å], generating an infinite two-dimensional network structure.


Related literature
As a result of their geometry, including the zigzag conformation of the spacer moiety (C-C and C N-N C) between the two terminal groups, double Schiff base compounds have proved to be very versatile in their ability to form novel frameworks by self-assembly reactions with metal salts, see: He et al. (2008). For BrÁ Á ÁBr interactions, see: Metrangolo et al. (2005).
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: ZS2037).
works by self-assembly reactions with metal salts (He et al., 2008). In these compounds, the central C-C and N-N bridges are rotationally flexible and the significance of the relative orientations of these terminal groups in self-assembly reactions has become a matter of increasing interest in recent literature.
The structure of the title compound, C 18 H 16 Br 2 N 4 (I) (Fig. 1) shows two parallel 4-bromophenyl groups connected by flexible C-C and C═N-N═C bonds, with the molecule having crystallographic inversion symmetry. All atoms in the molecule are coplanar resulting in a linear conformation. In the solid state, the title compound is stabilized by weak intermolecular Br···Br interactions [3.7992 (11) Å] (Metrangolo et al., 2005), linking the molecules down the b axial direction in the cell, generating an infinite two-dimensional network structure (Fig. 2).

Experimental
A mixture of 2,3-butanedione dihydrazone (0.57 g, 5.0 mmol) and 4-bromobenzaldehyde (1.85 g, 10.0 mmol) with 2 drops of formic acid in ethanol (60 ml) was stirred at room temperature for ca. 1 hour to generate the title compound as a yellow solid (2.15 g, 96% yield). Single crystals suitable for X-ray analysis were grown in dichloromethane by slow evaporation at room temperature.

Refinement
All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were placed in idealized positions and treated as riding, with C-H = 0.93 Å (CH), 0.96 Å (CH 3 ) and U iso (H) = 1.2 U eq (CH) and U iso (H) = 1.5 U eq (CH 3 ). Fig. 1. The molecular structure and atom numbering scheme for (I) with displacement ellipsoids drawn at the 50% probability level. For symmetry code (i): -x, -y+1, -z].

Special details
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.
supplementary materials sup-3 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 Rfactors(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.