Crystal structure of tris(trans-1,2-diaminocyclohexane-κ2 N,N′)cobalt(III) trichloride monohydrate

The first structure with coordinates of tris(trans-1,2-diaminocyclohexane)cobalt(III) chloride monohydrate in the space group I 2d is reported.


Chemical context
We are interested in the hydrogen-bonding patterns of various coordination complexes, especially those that incorporate optically active ligands, where the role of hydrogen bonding in the chiral discrimination between coordination complexes is important. As part of our studies, we prepared the title complex by the reaction of racemic trans-(R,R,S,S)-1,2-diaminocyclohexane with [Co(NH 3 ) 5 Cl] 2+ in aqueous solution at 323 K. The resulting complex is a racemic mixture and does not exhibit optical activity. Isolation of optically active forms is being undertaken.
This complex was first reported in 1937 (Jaeger & Bijkerk, 1937) and by optical crystallography and X-ray diffraction, the space group was determined to be P6 1 and/or P6 5 . There have been several, successive studies on this compound, and all are reported in a variety of space groups and configurations of the ligand (P6 1(5) : Jaeger & Bijkerk, 1937 ISSN 2056ISSN -9890 1973. Note: the 1983 Kobyashi article is a correction of the space group reported for the 1972 paper. The Andersen structure was a unit-cell determination and heavy-atom coordinate prediction based on powder diffraction data. No coordinates are available for that structure. Herein, we report the structural characterization of tris(trans-1,2 diaminocyclohexane)cobalt(III) chloride monohydrate in I42d.

Structural commentary
The cation crystallizes on a twofold rotation axis at [x, 0.25, 0.625], thus, only half of the cation is represented in the asymmetric unit (Fig. 1). One chloride is located on the twofold axis at [0.75, y, 0.875] and the remaining independent chloride anion occupies a general position within the lattice. The water molecule of crystallization is also in a general position, but was modeled as a partial occupancy species (vide infra). The 1,2-diaminocyclohexane ligands adopt a lel 3 , Á (,,) configuration with the (R,R)-ligand in the featured example. The cobalt atom adopts an octahedral coordination environment with only small distortions from an ideal geometry (Table 1). The 1,2-diaminocyclohexane ligands are unexceptional.

Comparison with previously reported structures
An inspection of the structure and comparison with the Marumo lel 3 complex gives an r.m.s. fit of 0.0706 for the cobalt and nitrogen atoms (Marumo et al., 1970;Macrae et al., 2006).
The predominant difference between the Marumo structure and that reported here is the molecular symmetry. The Marumo structure adopts C 3 symmetry, with only one unique ligand. The structure herein adopts C 2 symmetry with one complete and one half ligand in the asymmetric unit.
Perhaps the most surprising change when compared with the Andersen structure is the contraction in cell parameters and overall cell-volume reduction. The cell parameters reported by Andersen are a = 19.208, c = 13.908 Å , V = 5131.3 Å 3 (Andersen et al., 1973). Our study has a = 18.786, c = 13.857 Å and V = 4830.3 Å 3 . This change represents a 4.6% reduction in cell volume, with a and b contracting in a concerted fashion by nearly 0.5 Å . Typically one might expect a contraction of around 0.1 to 0.2 Å upon cooling, similar to that observed for the change in c. This observation led us to undertake variable temperature studies to determine if this Labeling scheme for (I). Atomic displacement parameters are depicted at the 50% probability level. [Symmetry code: (i) x, Ày + 1 2 , Àz + 5 4 .] Table 1 Selected geometric parameters (Å , ).

Figure 2
Packing diagram of (I), viewed along the c axis. Hydrogen bonds are shown as dashed lines.
was actually the case. Data on a crystal of the title compound were recorded at 120 K, 250 K and 293 K. Cell parameters and refinement statistics are given in Table 2. It should be noted that the redetermination of the unit cell at room temperature with a single crystal sample yielded a unit cell that is approximately 100 Å 3 smaller in volume than that calculated originally from powder diffraction data.

Supramolecular features
The complex forms a hydrogen-bonded network with the amino nitrogen atoms on the cation serving as donors to nearby chlorine atoms and the water molecule ( Fig. 2, Table 3). Although the water hydrogen atoms could not be located, there are contacts to nearby chlorine atoms from the oxygen atom at reasonable hydrogen-bond contact distances (Table 3). Close inspection of the Fourier difference map reveals several locations for potential hydrogen-atom sites on the water oxygen. However, none of these sites refines suitably when modeled as a hydrogen atom. Further exacerbating this situation is the disorder apparent with this lattice water molecule, because through symmetry there is another water oxygen atom located only 2.11 Å distant. Clearly this is unreasonable and reflects the disorder in this molecule. The water of crystallization and chlorine anions are arranged within discrete pockets within the lattice. Other contacts are simple van der Waals interactions.

Database survey
This structure was first reported in 1937 (Jaeger & Bijkerk, 1937) with the space group P6 1 and P6 5 at room temperature.
Other reports of the structure with the P6 1 space group were in 1970 (Marumo et al., 1970) and 1977 (Sato & Saito, 1977), both at room temperature. The structure was also reported in 1972 (Kobayashi et al., 1972) with the C2 space group and 1983 (Kobayashi et al., 1983) with the R32 space group. The first report of the structure with the I42d space group was in 1973 (Andersen et al., 1973). This structure is at room temperature and no coordinates were provided by the authors. The structure presented in this paper has the same I42d space group and provides coordinates for the structure at cryogenic temperatures.  Data recorded on a second crystal selected from same batch. Crystal showed signs of degradation at higher temperatures, presumably due to solvent loss. Inspection of the crystal after 293 K data set showed fracturing within the crystal. Table 3 Hydrogen-bond geometry (Å , ). (ii) y þ 1; Àx þ 1; Àz þ 1;

Synthesis and crystallization
0.56 g of [Co(NH 3 ) 5 Cl]Cl 2 was dissolved in 200 mL of DI water and allowed to stand overnight. 1.54 g of racemic trans-(R,R,S,S)1,2-diaminocyclohexane was added along with a small amount of charcoal. The mixture was stirred and heated at 313-323K for 2 d. The solution was filtered through a SP Sephadex C25 column. Using first 0.01 M HCl then 1 M HCl, the product was collected from the column. The fractions were placed in evaporation dishes and allowed to dry for three weeks. Orange crystals formed in the evaporation dish and were harvested for analysis.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. All non-hydrogen atoms were refined with anisotropic atomic displacement parameters. Hydrogen atoms were included in geometrically calculated positions with U iso (H) = 1.2U eq (C/N). C-H distances were fixed at 0.95 Å and N-H distances fixed at 0.91 Å . The water of crystallization was determined to be partially occupied by inspection of the displacement parameters during refinement. The occupancy was set to 50% in the final model which yielded reasonable displacement parameters. Hydrogen atoms could not be located or reliably modeled on the water molecule, but have been included in the chemical formula for completeness.  -1,2-diaminocyclohexane-κ 2 N,N′)

cobalt(III)
trichloride monohydrate Megan K. Gallagher, Allen G. Oliver and A. Graham Lappin

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