(R,R)-4,4′-Dibromo-2,2′-[cyclohexane-1,2-diylbis(nitrilomethylidyne)]diphenol

The molecule of the title compound, C20H20Br2N2O2, lies on a twofold axis. It contains two stereogenic C atoms with R chirality and thus it is the enatiomerically pure R,R-diastereomer. There is an intramolecular O—H⋯N hydrogen bond.

The molecule of the title compound, C 20 H 20 Br 2 N 2 O 2 , lies on a twofold axis. It contains two stereogenic C atoms with R chirality and thus it is the enatiomerically pure R,Rdiastereomer. There is an intramolecular O-HÁ Á ÁN hydrogen bond.
The molecular structure of (I) is built from two halves related through a two fold axix passing through the middle of the C8-C8 i and C10-C10 i bonds [(i)= 1-x, 2-y, z)] (Fig. 1). The stereogenic carbon C8 has the R chirality and so the molecule is the enantiomerically pure R,R diastereomer which confirms the synthetic patway used. This molecule is closely related to the (R,R)-N,N'-Bis(5-chlorosalicylidene)-1,2-cyclohexanediamine compound (Yang et al., 2004).
Intramolecular O-H···N hydrogen bonds also exist in this molecule and thus stabilize the structure (Table 1).

Experimental
The title compound was synthesized according to the literature (Yang et al., 2004) using the reaction of (R,R)-1,2-cyclohexanediamine, Na 2 SO 4 , and 5-bromon-2-hydroxybenzaldehyde under mild condition. (R,R)-4,4'-Bromo-2,2'-[cyclohexane-1,2-diylbis (nitrilomethylidyne)]diphenol (0.52 g, 1 mmol) was added to a solution of Cd(AC) 2 .4H 2 O(0.26g, 1mmol) in methanol(20mL). The mixture was heated for 20 hs under reflux with stirring. It was then filtered to give a clear solution, into which diethyl ether vapour was allowed to condense in a closed vessel. After being allowed to stand for a two weeks at room temperature, colorless single crystals were used to measure X-ray diffraction analysis.

Refinement
The absolute configuration has been deduced from the X-ray structural analyses and confirms the predicted configuration expected from the synthetic pathway.

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.
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 > 2sigma(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq