3,3′-Dibromo-4,4′-[(1R,2R)-cyclohexane-1,2-diyldiimino]dipent-3-en-2-one

The asymmetric unit of the title compound, C16H24Br2N2O2, contains two independent molecules, each which has two intramolecular N—H⋯O hydrogen bonds linking the amine N atoms to the enolic O atoms of the same acacH-imine unit. In the crystal, the molecules are lined up by intermolecular weak C—H⋯O hydrogen bonds, forming two vertical each other two-dimensional chains along the a axis and b axis of the unit cell, respectively.

The asymmetric unit of the title compound, C 16 H 24 Br 2 N 2 O 2 , contains two independent molecules, each which has two intramolecular N-HÁ Á ÁO hydrogen bonds linking the amine N atoms to the enolic O atoms of the same acacH-imine unit. In the crystal, the molecules are lined up by intermolecular weak C-HÁ Á ÁO hydrogen bonds, forming two vertical each other two-dimensional chains along the a axis and b axis of the unit cell, respectively.
The crystal structure of the title compound is shown in Fig. 1, each dissymmetrical unit cell contains two vertical each other independent molecules. Each molecule has two intramolecular N + -H···Ohydrogen bonds, which links each nitrogen atoms to the corresponding nearby terminal oxygen atoms of the same acacH-imine unit (N1-H1···O1, N2-H2···O2, N3-H3···O3 and N4-H4···O4, Table 1) such that a coplanar six-membered ring is generated. As shown in Fig. 2, the molecules of the title compound are lined up by the intermolecular interaction (C-H···O, Table 1.) forming two vertical each other two-dimensional chains along the a axis and b axis of the unit cell, respectively. The structure also shows a non-coplanar array for the (R, R)-cyclohexanediamine moiety and both of the C=N imine groups have the Z arrangements with respect to the chiral C-C sigma bond (C6-C11 or C22-C27) in the cyclohexanediamine, and the Schiff base molecule are non-coplanar due to chirality of the cyclohexanediamine moiety.

Experimental
1R,2R-Diaminocyclohexane (0.115 g, 1.00 mmol) was added slowly, whilst stirring, to a methanol (15 ml) solution with acetylacetone (0.2 g, 2.00 mmol), and the mixture was heated at reflux for 2 h. After cooling, and the solvent was removed under reduced pressure. The crude product was purified by column chromatography over silica gel using 20% EtOAc-hexane to afford pure yellow crystals of N,N'-bis-acetylacetone-1R,2R-dDiaminocyclohexane and dried in vacuum. Solid Nbromosuccimide (0.088 g, 0.5 mmol) was added slowly, whilst stirring, to a solution of the compound 1 (0.14 g, 0.5 mmol) in ethanol (20 ml). Stirring the solution for 2 h, and then the solvent was removed under reduced pressure. The crude product was purified by column chromatography over silica gel using 35% EtOAc-CH 2 Cl 2 to afford pure pale yellow crystals of 2 and dried in vacuum, 0.1 g (yield 46%). Single crystals suitable for X-ray diffraction were obtained from an ethanol-CH 2 Cl 2 mixture by slow evaporation at room temperature.

Refinement
All H atoms were placed in calculated positions and refined as riding, with C-H = 0.96-0.98 Å, N-H = 0.86 Å, and U iso (H) = 1.2-1.5U eq (C,N). Fig. 1. The molecular structure of (II) showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level. Dashed lines indicate hydrogen bonds.

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 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.