(1R*,2S*)-N,N′-Bis[(E)-1H-pyrrol-2-ylmethylidene]cyclohexane-1,2-diamine monohydrate

The title compound, C16H20N4·H2O, was synthesized from cis-1,2-diaminocyclohexane (a racemic mixture of the (1R,2S) and (1S,2R) enantiomers). The compound crystallized with two molecules (A and B) in the asymmetric unit with a single water solvent molecule per Schiff base molecule. Molecules A and B have similar conformations as illustrated by the least-squares-fit with an r.m.s. deviation of 0.242 Å. The molecules within the asymmetric unit are bridged by hydrogen bonds to the two water molecules, resulting in a heterotetramer. The water molecule acts as both a hydrogen-bond donor and acceptor. The pyrrole-imine units are not co-planar, making an angle of 73.9 (3)° and 76.9 (3)° in molecules A and B, respectively.

The title compound, C 16 H 20 N 4 ÁH 2 O, was synthesized from cis-1,2-diaminocyclohexane (a racemic mixture of the (1R,2S) and (1S,2R) enantiomers). The compound crystallized with two molecules (A and B) in the asymmetric unit with a single water solvent molecule per Schiff base molecule. Molecules A and B have similar conformations as illustrated by the least-squaresfit with an r.m.s. deviation of 0.242 Å . The molecules within the asymmetric unit are bridged by hydrogen bonds to the two water molecules, resulting in a heterotetramer. The water molecule acts as both a hydrogen-bond donor and acceptor. The pyrrole-imine units are not co-planar, making an angle of 73.9 (3) and 76.9 (3) in molecules A and B, respectively.

Related literature
For a study of the helical structures formed by both the S,S and R,R bis(pyrrolide-imine) ligands as well as the Zn II , Cu II and Ni II chelates in the solid state, see: Wang et al. (2007). For the solid-state synthesis and X-ray structure of the anhydrous trans racemate of the ligand, see: van den Ancker et al. (2006). For the Ti IV chelate of the trans racemic complex, see: Zhang et al. (2008). For the intermolecular interaction-controlled self-assembly and a study of the photophysics of the Pt II chelate of the R,R and S,S enantiomers as well as the trans racemic complex, see: Shan et al. (2008). For the X-ray structure and applications of the trans racemate of the Pd II chelate as a hydrogenation catalyst, see: Bacchi et al. (2003).  Table 1 Hydrogen-bond geometry (Å , ). I would like to thank the University of KwaZulu-Natal for the use of their facilities and the National Research Foundation (South Africa) for funding. structure of molecule A showing the atom numbering scheme is shown in Figure 1. The geometry of molecules A and B are very similar, this is illustrated by a least squares fit ( Figure 2) (Mercury, Macrae et al., 2006). The RMSD for the fit is 0.242 Å. The fit shows that the biggest difference between the two structures is the torsion angle of the pyrrole rings relative to the cyclohexane linkage. The C6-N2-C4-N1 torsion angle is 179.7 (2) and 167.0 (2)° for molecules A and B, respectively. The C11-N3-C16-N4 torsion angle measures 173.5 (2) and 178.9 (2)° for molecules A and B, respectively. The mean imine C=N bond lengths are 1.270 (4) and 1.269 (3) Å for molecules A and B, respectively. These bond lengths highlight the double bond character of the imine bond. The pyrrole-imine moieties of both molecules A and B in the asymmetric unit are not co-planar. The angle subtended by the two seven atom mean planes comprising the pyrrole ring and imine carbon and nitrogen atoms is 73.9 (3)° and 76.9 (3)° for molecules A and B, respectively. This angle allows for hydrogen bonding to two water molecules. Both the imine nitrogen atoms and the pyrrole NH groups are involved in the hydrogen bonding, giving a total of eight hydrogen bonds. The hydrogen bonds result in a water-bridged dimer structure (Figure 3). The hydrogen bonds are considerably shorter than the sum of the van der Waals radii and the bond angles are approaching ideality, suggesting that they are likely to be relatively strong interactions. The hydrogen bond lengths and bond angles are summarized in Table 1.

Experimental
The enatiomerically pure diamine, (1R,2S)-diaminocyclohexane, (0.303 g, 2.65 mmol) was ground in an agate pestle and mortar with pyrrole-2-carboxaldehyde (0.500 g, 5.30 mmol) for 10 minutes. The resulting brown oil was dissolved in dichloromethane and dried over magnesium sulfate to remove the water, a by-product from the condensation reaction. The dichloromethane solution was then concentrated and layered with hexane to re-crystallize the ligand by liquid-liquid difussion (0.512 g, 72% yield). Crystals suitable for single-crystal X-ray crystallography, were obtained from the crystallization process.

Refinement
The positions of all C-bonded hydrogen atoms were calculated using the standard riding model of SHELXL97 (Sheldrick, 2008) with C-H(aromatic) distances of 0.95 Å and U iso = 1.2 U eq , C-H(methylene) distances of 0.99 Å and U iso = 1.2 U eq and a C-H(methine) distance of 1.00 Å and U iso = 1.2 U eq . The pyrrole NH atoms and the hydrogen atoms of the water molecules were located in the difference density map and allowed to refine isotropically.

Figure 1
Thermal ellipsoid plot of molecule A of (1), rendered at 30% probability. Hydrogen atoms are shown as spheres of arbitrary radius. The solvent molecules and the second molecule of the asymmetric unit have been omitted for clarity.     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.  (3) 155 (2)