Synthesis and structure determination of racemic (Δ/Λ)-tris(ethylenediamine)cobalt(III) trichloride hemi(hexaaquasodium chloride)

In the racemic title compound, the [Co(en)3]3+ and [Na(H2O)6]+cations exist in distorted octahedral coordination environments and charge neutrality in the salt is furnished by Cl− anions. Structural cohesion is maintained by an array of C—H⋯O, N—H⋯Cl and O—H⋯Cl hydrogen bonds.


Chemical context
The coordination complex-cation tris-ethylenediamine cobalt(III), [Co(en) 3 ] 3+ , was influential in Werner's development of the structure of transition-metal complexes as it could be resolved into its two enantiomers by selective crystallization using tartrate anions, thus helping to demonstrate the octahedral geometry of the metal ion (Werner, 1912). As such, the synthesis of members of this family of complexes is a common undergraduate laboratory experiment (Work & McReynolds, 1946;Broomhead et al., 1960;Girolami et al., 1999;McClellan & Cass, 2015).
The synthesis and structural characterization of many members of this family of complexes, both racemic and resolved, have been undertaken over the years. In all cases, the [Co(en) 3 ] 3+ complex cation was found to have trigonally distorted octahedral symmetry, as expected, and the structures usually have significant hydrogen-bonding interactions involving the ethylene diamine ligands, the water molecules of hydration, and the anions present. ISSN 2056-9890

Structural commentary
The title compound crystallizes in the centrosymmetric trigonal space group P3c1 (Fig. 1). The asymmetric unit consists of a trivalent cobalt atom residing on a threefold axis chelated by an ethylene diamine (en; C 2 H 8 N 2 ) ligand. Two Cl anions, one occupying a general position and the other lying on a 3 axis are also present. One Na cation, also positioned on a 3 axis, with a water molecule (general position) bound to it are also observed. After application of crystal symmetry, the [Co(en) 3 ] 3+ and [Na(H 2 O) 6 ] + cationic complexes that result each adopt distorted octahedral geometries.
Within the chelating en ligand, given the sp 3 -hybridization of the C atoms and an expected tetrahedral coordination environment around those C atoms, bond angles around each should be near the expected 109.5 . The values obtained from the crystal structure [minimum = 106.33 (15) (Nakatsu et al., 1957;Farrugia et al., 2000), where the sodium cations and chloride anions showed signs of disorder, no features suggestive of disorder are observed in the structure of the racemate.

Supramolecular features
The en-chelated, trivalent cobalt atom in the title compound lies on a threefold axis housed within the (021) plane. As a result of crystal symmetry, the full [Co(en) 3 ] 3+ cation is generated and shows both the Ã and the Á configurations with distorted octahedral geometry (Jensen, 1970). By virtue of its residing on a threefold axis, the net +1 charge that results from the Co atom is balanced by a fully occupied Cl anion occupying a general position. Typically, changes in conformation of the en ligand can be attributed to hydrogen bonding; however, all en conformations in both Co(en) 3 and Cr(en) 3 cations demonstrate similar energies (Veal & Hodgson, 1972;Enemark et al., 1970;Raymond et al., 1968a,b;Raymond & Ibers, 1968). Analogous to the many structures encompassing the [Co(en) 3 ] 3+ cation, hydrogen-bonded arrays are prevalent in the solid-state structure between the en ligands and both water molecules and chloride anions (Table 1).
It is notable that Cl2 accepts six, symmetry-equivalent O1-H1EÁ Á ÁCl2 hydrogen bonds (Table 1) and forms a distorted Cl(H 2 O) 6 octahedron. Along the c-axis, the orientation of the sodium and Cl2 octahedra with respect to one another forms a herringbone-type pattern when looking into the ac plane ( Fig. 2). Collectively, the symmetry elements within the solid- Face-sharing octahedra looking into the ac plane encompassing the Na(H 2 O) 6 + cations and six water molecules hydrogen bonded to Cl2 that form a herringbone pattern with respect to one another. Anisotropic displacement ellipsoids have been set to the 50% probability level. Dashed lines represent hydrogen bonds.

Figure 1
Anisotropic displacement ellipsoid plot of 1 with ellipsoids set to the 50% probability level. Atoms in the asymmetric unit are labeled. Dashed lines represent hydrogen bonds.

Figure 3
Packing of 1 relative to the ab plane. Collectively, the symmetry elements make the structure of 1 an excellent example of the p6mm twodimensional space group. Anisotropic displacement ellipsoids have been set to the 50% probability level. state structure of the racemate make it an excellent illustration of the p6mm two-dimensional space group when looking towards the ab plane (Fig. 3).

Database survey
The structure of racemic [Co(en) 3 ]Cl 3 was reported to have the trigonal space group P3c1; however, no additional structural details were reported (Dingle & Ballhausen, 1967). This salt was later crystallized as the non-stoichiometric hydrate (2.8 water molecules per cobalt center) with long chains of hydrogen-bonded water molecules that precluded interactions between the incorporated water molecules and the ethylene diamine ligands (Whuler et al., 1975). This same salt was later crystallized as the tetrahydrate and included a one-dimensional water chain perpendicular to the [001] direction. The solid was vacuum dried to form void channels that could incorporate guest molecules (Takamizawa et al., 2008).
Like racemic [Co(en) 3 ]Cl 3 , other chemically-similar salts have been crystallized that demonstrate hydrogen-bonding arrays involving the ethylenediamine ligands, interstitial water molecules, and the counter-ions. These included the Ã-enantiomer of the monohydrate Cl and I salts, which crystallize in the tetragonal space group P4 3 2 1 2, in 1969 and 2001, respectively (Iwata et al., 1969;Matsuki et al., 2001). The structure of the bromide salt of the Á-enantiomer, Á-[Co(en) 3 ]Br 3 ÁH 2 O, was carried out in 1962, though the absolute structure could not be determined by anomalous dispersion at that time (Nakatsu, 1962). The structure of the right-handed helical enantiomer, Á-[Co(en) 3 ]I 3 ÁH 2 O was finally reported in 2019 and crystallizes in the orthorhombic space group P2 1 2 1 2 1 (Grant et al., 2019).
In this latter-most structure, it has been proposed that the significant hydrogen bonding involving the en ligands, the counter-ion and the water molecules of hydration is directly responsible for this material's circular dichroism spectrum (Raymond & Duesler, 1971).
As the student laboratory preparation usually involves the synthesis of the racemic double salt [Co(en) 3 ]ÁCl 3 Á0.5NaClÁ-3H 2 O (McClellan & Cass, 2015;Girolami et al., 1999), we were surprised to not find its structure reported in the Cambridge Structural Database. As mentioned previously, the structure of the Ã-enantiomer of the complex was first reported in 1957 (Nakatsu et al., 1957) and later redetermined in 2000 (Farrugia et al., 2000).

Refinement
Crystal data, data collection and structure refinement details for 1 are summarized in Table 2. With the exception of atom H1E, which was constrained to ride on the water O atom (O1), all other H atoms were located in the difference-Fourier map and freely refined with 0.91 < C-H < 0.99 Å , 0.81 < N-H < 0.84 Å , and O-H = 0.89 Å .

(Δ/Λ)-Tris(ethylenediamine)cobalt(III) trichloride hemi(hexaaquasodium chloride)
Crystal data 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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )