4,4′-Dibromo-2,2′-{ethane-1,2-diylbis[(methylimino)methylene]}diphenol

The asymmetric unit of the title compound, C18H22Br2N2O2, contains one half-molecule that is related to the other half by a center of inversion located at the mid-point of the central C—C bond. The hydroxy (phenolic) groups are linked to the N atoms by O—H⋯N hydrogen bonds, which generate S(6) rings.

The asymmetric unit of the title compound, C 18 H 22 Br 2 N 2 O 2 , contains one half-molecule that is related to the other half by a center of inversion located at the mid-point of the central C-C bond. The hydroxy (phenolic) groups are linked to the N atoms by O-HÁ Á ÁN hydrogen bonds, which generate S(6) rings.

Related literature
For the synthesis, see: Rivera et al. (2010). For the uses of tetrahydrosalens in coordination chemistry, see: Atwood (1997). For a related structure, see: Nazarenko et al. (2000). For reference bond lenghts, see: Allen et al. (1987).  Table 1 Hydrogen-bond geometry (Å , ). (19) 154 (2) Data collection: CrysAlis PRO (Oxford Diffraction, 2009); cell refinement: CrysAlis PRO; data reduction: CrysAlis PRO; program(s) used to solve structure: SIR2002 (Burla et al., 2003); program(s) used to refine structure: JANA2006 (Petříček et al., 2006); molecular graphics: DIAMOND (Brandenburg & Putz, 2005); software used to prepare material for publication: JANA2006.  Recently, we reported the synthesis of a new class of ligands by a ring-opening reduction of bis-1,3-benzoxazines with sodium borohydride (Rivera et al., 2010), and the products of these reactions are referred to as N,N'-disubstituted tetrahydro-salens (Atwood, 1997). Here we report the crystal structure of title compound (I). The C(sp 3 )-X bond distances and angles in (I) are within normal ranges (Allen et al., 1987) and comparable with a related structure (Nazarenko, et al., 2000). The C-N bonds in the N-CH 2 CH 2 -N segment are anti to each other, with a torsion angle of 180°. The observed conformation is stabilized by the short intramolecular hydrogen bonds O-H··· N (Table 1), and these interactions generate S(6) ring motifs.

Refinement
All hydrogen atoms were discernible in difference Fourier maps and could be refined to reasonable geometry. According to common practice H atoms bonded C atoms were kept in ideal positions with C-H distance 0.96 Å during the refinement.
The methyl H atoms were allowed to rotate freely about the adjacent C-C bonds. The hydroxy hydrogen was found in difference Fourier maps and its coordinates were refined freely. The isotropic atomic displacement parameters of hydrogen atoms were evaluated as 1.2×U eq of the parent atom. Fig. 1. The molecule of the title compound. Displacement ellipsoids are drawn at 50% probability level. Atoms with suffix i are generated by the symmetry operation (1-x, -y, 2-z).  Refinement. The refinement was carried out against all reflections. The conventional R-factor is always based on F. The goodness of fit as well as the weighted R-factor are based on F and F 2 for refinement carried out on F and F 2 , respectively. The threshold expression is used only for calculating R-factors etc. and it is not relevant to the choice of reflections for refinement.

Figures
The program used for refinement, Jana2006, uses the weighting scheme based on the experimental expectations, see _refine_ls_weighting_details, that does not force S to be one. Therefore the values of S are usually larger than the ones from the SHELX program.