Crystal structure of trans-cyclohexane-1,2-diammonium chromate(VI) from synchrotron X-ray diffraction data

In the title compound, (C6H16N2)[CrO4], the trans-cyclohexane-1,2-diammonium cations and chromate anions are connected through N—H⋯O hydrogen bonds. The tetrahedral CrO4 2− anion is slightly distorted due to the influence of the hydrogen bonds.

The structure of the title hybrid compound, (C 6 H 16 N 2 )[CrO 4 ], has been determined from synchrotron data. The organic cation adopts a chair conformation. The inorganic CrO 4 2À anion is slightly distorted owing to its involvement in N-HÁ Á ÁO hydrogen-bonding interactions with neighbouring trans-cyclohexane-1,2-diammonium cations, whereby the two Cr-O bonds to the O atoms acting as acceptor atoms for two hydrogen bonds are slightly longer than the other two Cr-O bonds for which only one acceptor interaction per O atom is observed. In the crystal, cations and anions are packed into layers parallel to (001), held together through the aforementioned N-HÁ Á ÁO hydrogen bonds.

Structural commentary
The bond lengths and angles are very similar than in the structure of the bis(3 0 -nitro-trans-cinnamate) compound with the same cation (Hosomi et al., 2000). The cyclohexane ring C-C bond lengths and angles and the torsion angles involving the C and N atoms are in essential agreement with the values obtained for [Cr(chxn) 3 ](ZnCl 4 )ClÁ3H 2 O (Moon & Choi, 2016

Supramolecular features
In the crystal structure, the cations and anions are arranged in layers parallel to (001). The ammonium group is directed towards the anion, hence causing polar and non-polar sections in the crystal structure, alternating along [001]. As mentioned above, each of the O atoms is involved in N-HÁ Á ÁO hydrogen bonds that hold the polar (001) sheets together (Fig. 2, Table 2).

Database survey
A search of the Cambridge Structural Database (Version 5.37, Feb 2016 with three updates; Groom et al., 2016) indicates a total of 31 hits for compounds containing the cyclohexanediammonium cation (C 6 H 16 N 2 ) 2+ .

Synthesis and crystallization
Compound (I) was prepared by dissolving 5 mmol of chromium trioxide (0.50 g, Sigma-Aldrich) and 0.5 mmol of trans- The molecular structures of the organic cation and the inorganic anion in (I), drawn with displacement ellipsoids at the 30% probability level. The dashed line represents a hydrogen-bonding interaction. Table 1 Selected geometric parameters (Å , ).
1,2-cyclohexanediamine (0.6 mL, Sigma-Aldrich) in 40 mL of distilled water with a molar ratio of 1:1. The mixture was stirred for 30 minutes and the resulting solution was allowed to stand at room temperature for one day to give plate-like yellow crystals suitable for X-ray structural analysis.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All H atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms, with C-H = 0.99-1.00 Å and N-H = 0.91 Å , and with U iso (H) values of 1.2 or 1.5U eq of the parent atoms.

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.