Crystal structure, Hirshfeld surface analysis and energy framework calculation of the first oxoanion salt containing 1,3-cyclohexanebis(methylammonium): [3-(azaniumylmethyl)cyclohexyl]methanaminium dinitrate

In the organic cation of the title salt, the cyclohexane ring is in chair conformation with the two methylammonium substituents in the equatorial positions. The crystal structure features extensive hydrogen-bonding interactions.


Chemical context
The design of new organic-inorganic hybrid ionic materials is of current interest for various applications, particularly in the areas of crystal engineering, supramolecular chemistry and materials science (Kimizuka & Kunitake, 1996;Mitzi et al., 1999;Bonhomme & Kanatzidis, 1998;Wachhold & Kanatzidis, 2000), and also for optical semiconductor materials (Kagan et al., 1999;Li et al., 2008). Among these hybrid compounds, organic nitrates are particularly interesting for their multiple applications including as catalytic precursors of numerous reactions, in biological treatment systems or as pharmacological products (Brandá n, 2012a(Brandá n, ,b, 2015Castillo et al., 2011;Torfgå rd & Ahlner, 1994).
1,3-Cyclohexanebis(methylamine) (CHMA) is used industrially as a hardener for epoxy resins, a raw material for the production of polyamides and isocyanates, a rubber chemical for paper-processing agents, in fiber treatment agents and in cleaning agents (Pham & Marks, 2012). It can also be used as an effective new cross-linking agent for the chemical modification of polyimide membranes (Shao et al., 2005). We have previously reported on the use of the 1,3-cyclohexanebis(methylammonium) dication in the syntheses of organicinorganic hybrid ionic complexes (Huo et al., 1992;Yang et al., 2008), but to the best of our knowledge there are no reported salt forms containing an oxoanion and 1,3-cyclohexanebis(methylammonium). ISSN 2056-9890 In a continuation of our recent studies of new hybrid compounds containing an organic cation and an inorganic oxoanion such as CrO 4 2À (Chebbi et al., 2000;Chebbi & Driss, 2001, 2002a,b, 2004, Cr 2 O 7 2À (Chebbi et al., 2016;Ben Smail et al., 2017), NO 3 À (Chebbi et al., 2014) and ClO 4 À Ben Jomaa et al., 2018), we report in this work the crystal structure, Hirshfeld surface analysis and energyframework calculations for a new organic nitrate, (C 8 H 20 N 2 )[NO 3 ] 2 (I).

Structural commentary
The title compound crystallizes with one 1,3-cyclohexanebis(methylammonium) dication, (CHMA) 2+ , and two nitrate anions in the asymmetric unit (Fig. 1). An EDX spectrum confirming the presence of C, N, and O is shown in Fig. 2.
All atoms of the nitrate anion are coplanar with the N-O bond distances varying between 1.218 (4) and 1.250 (4) Å . The O-N-O angles are in the range of 118.8 (4)-121.1 (4) . These bond lengths and angles are in good agreement with those observed in similar compounds (Blerk & Kruger, 2009;Gatfaoui et al., 2017;Hakiri et al., 2018). The cyclohexane ring of the organic cation adopts a chair conformation with the methylammonium substituents in the equatorial positions and the two terminal ammonium groups in a trans conformation (Fig. 1). The same trans conformation has been observed in other compounds with this organic cation (Zhao et al., 2008). Examination of the C-C (N) distances and C-C-C (N) angles in the (CHMA) 2+ dication shows no significant difference from those in other organic materials associated with the same organic groups (Huo et al., 1992;Yang et al., 2008).

Supramolecular features
In the crystal, the (CHMA) 2+ cations and nitrate anions are linked by N-HÁ Á ÁO hydrogen bonds into infinite layers parallel to the ac plane (Fig. 3). Each layer is formed of infinite undulating chains running parallel to the [001] direction, further extended into an overall two-dimensional supramolecular network structure (Fig. 4). As seen in Table 1, all of the hydrogen atoms bonded to the amine group of (CHMA) 2+ dication contribute to the formation of N-HÁ Á ÁO hydrogen bonds with the nitrate anions. The -N(1,2)H 3 + groups of the organic cations, which act as donors to the N(3,4)O 3 À anions generate R 2 1 (3) dimeric rings. The propagation of these dimers produces infinite undulating chains running parallel to the The EDX spectrum of (I), showing the presence of C, N, and O.

Hirshfeld surface analysis and energy framework calculations
The Hirshfeld surfaces (Spackman & Jayatilaka, 2009) and their relative 2D fingerprint plots (Spackman & McKinnon, 2002;Parkin et al., 2007;Rohl et al., 2008)) were drawn using CrystalExplorer 3.1 (Wolff et al., 2012). The quantifying and decoding of the intermolecular contacts in the crystal packing are visualized using d norm (normalized contact distance) and 2D fingerprint plots, respectively. The dark-red spots on the d norm surface arise as a result of short interatomic contacts, while the other intermolecular interactions appear as light-red spots. d i (inside) and d e (outside) represent the distances to the Hirshfeld surface from the nuclei, with respect to the relative van der Waals radii. The proportional contribution of the contacts over the surface is visualized by the color gradient (blue to red) in the fingerprint plots.

Figure 4
Unit-cell contents for (I) shown in projection down the a axis, showing the infinite undulating chains. The orange dotted lines indicate N-HÁ Á ÁO hydrogen bonds.

Figure 6
A view of the supramolecular layer in the ac plane of (I), showing the formation of the R 2 1 (3), R 6 3 (12) and R 6 4 (14) motifs. C and H atoms not involved in hydrogen bonds (orange dashed lines) have been omitted for clarity.
The intermolecular interactions present in the structure are also visible on the two-dimensional fingerprint plot, which can be decomposed to quantify the individual contributions of each intermolecular interaction involved in the structure. The HÁ Á ÁO/OÁ Á ÁH contacts associated with N-HÁ Á ÁO hydrogen bonding appear to be the major contributor to the crystal packing (68.8%); these contacts are represented by the spikes in the top-left (d e > d i , HÁ Á ÁO, 31.3%) and bottom-right (d e < d i , OÁ Á ÁH, 37.5%) regions of the related plots in Fig. 8a. Interactions of the type HÁ Á ÁH appear in the middle of the scattered points in the fingerprint plots; they comprise 24.6% of the entire surface (Fig. 8b). The forceps-like tips in the region d e + d i ' 3 Å of the fingerprint plot (Fig. 8c) represent a significant HÁ Á ÁN/NÁ Á ÁH contribution, covering 4.2% of the total Hirshfeld surface of (I). The OÁ Á ÁO contacts, which represent only 2.4% of the Hirshfeld surface, Fig. 8d, are extremely impoverished in the crystal (enrichment ratio E OO = 0.17; Jelsch et al., 2014), as the oxygen atoms bound to nitrogen and the NO 3 group as a whole are electronegative, therefore the OÁ Á ÁO contacts are electrostatically repulsive. Fig. 9 shows the voids (Wolff et al., 2012) in the crystal structure of (I). These are based on the sum of spherical atomic electron densities at the appropriate nuclear positions (procrystal electron density). The crystal-void calculation (results under 0.002 a.u. isovalue) shows the void volume of title compound to be of the order of 469.14 Å 3 and surface area in the order of 1334.82 Å 2 . With the porosity, the calculated void volume of (I) is 17%. There are no large cavities. We note that the electron-density isosurfaces are not completely closed around the components, but are open at those locations where interspecies approaches are found, e.g. N-HÁ Á ÁO.
The crystallographic information file (crystal geometry and hydrogen bond distances to 1.083 Å ) was used as input to CrystalExplorer 17 (Turner et al., 2017) and the intermolecular interaction energies were calculated for the energy-framework analysis. This calculation is estimated from a single-point molecular wavefunction at B3LYP/6-31G(d,p). A cluster of radius 3.8 Å was generated around the molecule and the energy calculation was performed. The neighbouring molecules (density matrices) are generated within this shell by applying crystallographic symmetry operations with respect to the central molecule (density matrix). The interaction energy is broken down as Void plot for (I).

Figure 7
Hirshfeld surface around the constituents of (I) coloured according to d norm . The surface is shown as transparent to allow visualization of the orientation and conformation of the functional groups.   Table 2 shows the results of the interaction energies calculations. The results are represented graphically in Fig. 10 as framework energy diagrams. The molecular pair-wise interaction energies calculated for the construction of energy frameworks are used to evaluate the net interaction energies. The total interaction energies are electrostatic (E 0 ele = À247.8 kJ mol À1 ), polarization (E 0 pol = À379.5 kJ mol À1 ), dispersion (E 0 dis = À43.7 kJ mol À1 ), repulsion (E 0 rep = 23.6 kJ mol À1 ), and total interaction energy (E tot = À566.3 kJ mol À1 ). The electrostatic energy framework is dominant over the dispersion energy framework (Fig. 10).

Synthesis and crystallization
1,3-Cyclohexanebis(methylamine) (1 mmol) was dissolved in water (10 mL) and nitric acid (2 mmol in 10 mL of water). The resulting solution was stirred for 3 h, filtered and then left to stand at room temperature. Colorless crystals were obtained after five days by slow evaporation.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All C-bound hydrogen atoms were included in calculated positions with C-H = 0.98 (CH group) or 0.97 Å (methylene) and allowed to ride, with U iso (H) = 1.2U eq (C). N-bound H atoms were located in difference-Fourier maps and freely refined. Energy framework diagram for separate electrostatic (left, red) and dispersion (middle, green) components of (I) and the total interaction energy (right, blue). The energy factor scale is 60 and the cut-off is 5.00 kJ mol À1 . Table 2 Interaction energies.
N refers to the number of molecules with an R molecular centroid-to-centroid distance (Å ). Energies are in kJ mol À1 .