Dimethyl 4,4′-(diazenediyl)dibenzoate at 100 K

In the asymmetric part of the unit cell of the title compound, C16H14N2O4, there are two chemically equivalent but crystallographic independent half molecules. The geometric centre of each complete molecule lies on a crystallographic inversion centre. Both molecules are almost planar [mean deviations of atoms in the two molecules are 0.032 (2) and 0.044 (2) Å] and their geometries are similar. In the crystal, molecules are arranged in columns along the a axis. There are no intermolecular donor–acceptor distances shorter than 3.4 Å.

In the asymmetric part of the unit cell of the title compound, C 16 H 14 N 2 O 4 , there are two chemically equivalent but crystallographic independent half molecules. The geometric centre of each complete molecule lies on a crystallographic inversion centre. Both molecules are almost planar [mean deviations of atoms in the two molecules are 0.032 (2) and 0.044 (2) Å ] and their geometries are similar. In the crystal, molecules are arranged in columns along the a axis. There are no intermolecular donor-acceptor distances shorter than 3.4 Å .

Related literature
For general background to the use of azo compounds as dyes, pigments and advanced materials, see: Allmann (1997)

Comment
Azo compounds and its derivatives represent the dominant class of coloured compounds and are used as dyes and pigments (Allmann, 1997). Azobenzenes, due to its efficient and fully reversible photoisomerization and photoinduced anisotropy have widely been investigated as a component in photoresponsive materials (Zeitouny et al., 2009).
There are two chemically equivalent but crystallographic independent molecules in the unit cell ( Figure 1), both lie on crystallographic inversion centres. The molecules are almost planar and their geometries are similar; differences do not exceed 0.2 Å for bond lengths, 2° for valence angles and 3° for torsion angles.
All bond distances and angles lie in expected ranges and are in good agreement with the geometry of other parasubstituted azobenzene derivatives (Yu & Liu, 2009;Niu et al., 2011 andAllen, 2002).
The crystal packing is shown in Fig. 2. The molecules form columns along the a axis, the distance between stacked molecules is equal to 3.8146 (8) Å. There are no intermolecular C-H···O/N distances shorter than 3.4 Å.

Experimental
The compound was prepared according to literature procedure, (Onto et al., 1998). Crystals of (I) suitable for X-ray crystal structure analysis was grown from benzene-n-heptane mixture (1:1).

Refinement
Apart from methyl hydrogens all H atoms were positioned geometrically, with C-H = 0.93 Å and U iso (H) = 1.2U eq (C), methyl H atoms were derived from difference Fourier maps and refined as idealized groups with with C-H = 0.96 Å and U iso (H) = 1.5U eq (C). All methyl-H atoms were allowed to rotate but not to tip.  The molecular structure showing 50% displacement ellipsoids (arbitrary spheres for the H atoms). Symmetry code: A = -

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. 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.