Redetermination of (E)-N,N′-bis(4-bromophenyl)formamidine

In comprison with the previous structural study [Anulewicz et al. (1991 ▶). Pol. J. Chem. 65, 465–471], for which only the coordinates of all non-H atoms and of some H atoms were reported, the current redetermination of the title compound, C13H10Br2N2, additionally reports anisotropic displacement parameters for all non-H atoms and the coordinates of all H atoms, accompanied by higher accuracy of the geometric parameters. Two independent half-molecules are present in the asymmetric unit, which are completed by a twofold rotation axis as symmetry element. In the crystal, intermolecular N—H⋯N hydrogen bonds link the molecules into dimers. Linear chains parallel to [102] are formed by intermolecular Br⋯Br interactions of 3.4328 (7) Å between two Br atoms of adjacent molecules. The dihedral angles between the benzene rings are 50.05 (15) and 75.61 (11)° in the two independent molecules. Owing to the twofold symmetry of the molecules, H atoms attached to the N atoms are only half-occupied, leading to them being disordered over two positions of equal occupancy.

In comprison with the previous structural study [Anulewicz et al. (1991). Pol. J. Chem. 65,[465][466][467][468][469][470][471], for which only the coordinates of all non-H atoms and of some H atoms were reported, the current redetermination of the title compound, C 13 H 10 Br 2 N 2 , additionally reports anisotropic displacement parameters for all non-H atoms and the coordinates of all H atoms, accompanied by higher accuracy of the geometric parameters. Two independent half-molecules are present in the asymmetric unit, which are completed by a twofold rotation axis as symmetry element. In the crystal, intermolecular N-HÁ Á ÁN hydrogen bonds link the molecules into dimers. Linear chains parallel to [102] are formed by intermolecular BrÁ Á ÁBr interactions of 3.4328 (7) Å between two Br atoms of adjacent molecules. The dihedral angles between the benzene rings are 50.05 (15) and 75.61 (11) in the two independent molecules. Owing to the twofold symmetry of the molecules, H atoms attached to the N atoms are only half-occupied, leading to them being disordered over two positions of equal occupancy.

Comment
With the determination of reliable intermolecular distances, Br···Br interactions (Fujiwara et al., 2006;Reddy et al., 1996) and N-H···N hydrogen bonding (Del Bene & Elguero, 2006;Grotjahn et al. 2000;Thar & Kirchner, 2006) became important criteria in the description of supramolecular chemistry and in applied crystal engineering. The title compound C 13 H 10 Br 2 N 2 , (I), has been determined previously by Anulewicz et al. (1991). However, in that study only coordinates of all non-H atoms and of some H atoms were given. The present re-determination additionally reports anisotropic displacement parameters for all non-H atoms and the coordinates of all H atoms, accompanied by higher accuracy of all geometric parameters.
In (I) two independent half-molecules are present in the asymmetric unit which are completed by a twofold rotation axis as symmetry element that runs to the central C-H groups (C1-H1 and C2-H2, respectively). One molecule is displayed in Fig. 1. The dihedral angles between the two benzene rings in the individual molecules are 50.05 (15) ° for the first and and 75.61 (11) ° for the second molecule.
In the crystal, intermolecular N-H···N hydrogen bonds link the individual molecules into dimers (Fig. 2). Linear chains parallel to [102] are formed by intermolecular Br···Br interactions of 3.4328 (7) Å between two bromine atoms of adjacent molecules (Fig. 3). This interaction is significantly less than the van der Waals contact of 3.90 Å (Reddy et al., 1996;Fujiwara et al., 2006), hence making this interaction important for consolidation of the crystal packing.

Experimental
The title compound was synthesized by the following reaction. 17.202 g (0.1 mol) of 4-bromobenzenamine and 8.33 ml (0.05 mol) of triethyl orthoformate were combined in a round-bottom flask equipped with a distillation tube and heated at 160 until the distillation of ethanol creased. The retained solid was washed with ether, and dried under a dynamic vacuum to yield 16.10 g of white solid, (91%). 0.04 g of the white solid was dissolved in THF (3 ml

Refinement
H atoms attached to C atoms were positioned geometrically with C-H = 0.93 (CH), and constrained to ride on their parent atoms, with U iso (H) = 1.2U eq (C). H atoms attached to N atoms were found from difference Fourier maps and were fixed.
They were refined with U iso (H) = 1.2U eq (N). Owing to the 2 symmetry of the molecules, the H atoms attached to the N atoms are only half-occupied, leading to being disordered over two positions of equal occupancy.