Crystal structure of N,N′-bis(pyridin-4-ylmethyl)cyclohexane-1,4-diammonium dichloride dihydrate

In the title salt, C18H26N4 2+2·Cl−·2H2O, the N,N-bis(pyridin-4-ylmethyl)cyclohexane-1,4-diammonium dication lies on a crystallographic inversion center, and the central cyclohexyl ring adopts a chair conformation. In the crystal, dications, anions and solvent water molecules are connected via N/C/O—H⋯Cl and N—H⋯O hydrogen bonds and C—H⋯π interactions, forming a three-dimensional network.

Treatment of N,N-bis(pyridin-4-ylmethyl)cyclohexane-1,4-diamine with hydrochloric acid in ethanol led to the formation of the title salt, C 18 H 26 N 4 2+ Á2Cl À Á2H 2 O, which lies about a crystallographic inversion center at the center of the cyclohexyl ring. The asymmetric unit therefore comprises one half of the N,N-bis(pyridin-4-ylmethyl)cyclohexane-1,4-diammonium dication, a chloride anion, and a solvent water molecule. In the dication, the two trans-(4pyridine)-CH 2 -NH 2 -moieties occupy equatorial sites at the 1-and 4-positions of the central cyclohexyl ring, which is in a chair conformation. The terminal pyridine ring is tilted by 27.98 (5) with respect to the mean plane of the central cyclohexyl moiety (r.m.s. deviation = 0.2379 Å ). In the crystal, dications, anions, and solvent water molecules are connected via N/C/O-HÁ Á ÁCl and N-HÁ Á ÁO hydrogen bonds together with C-HÁ Á Á interactions, forming a threedimensional network.

Chemical context
An enormous number of metal-organic frameworks (MOFs) have been developed over the last two decades because of their attractive topologies and their desirable applications in a wide range of fields (Silva et al., 2015;Furukawa et al., 2014). For the development of these MOFs, many chemists have designed and prepared various dipyridyl-type ligands (Robin & Fromm, 2006;Robson, 2008;Leong & Vittal, 2011). Our group has also focused on the search for extended dipyridyltype ligands with a bulky central section for the development of MOFs with intriguing topologies or useful properties. As a part of our ongoing efforts, we prepared just such a dipyridyltype ligand with a central cyclohexyl moiety, namely N,Nbis(pyridin-4-ylmethyl)cyclohexane-1,4-diamine, synthesized by a condensation reaction between 1,4-cyclohexanediamine and 4-pyridinecarboxaldehyde according to a literature procedure (Huh & Lee, 2007). Herein we report on the crystal structure of the title salt obtained by the protonation of both amine groups in this molecule. ISSN 2056-9890

Structural commentary
The asymmetric unit of the centrosymmetric title salt, C 18 H 26 N 4 2+. 2Cl À. 2H 2 O, comprises one half of N,N-bis(pyridin-4-ylmethyl)cyclohexane-1,4-diammonium dication, a chloride anion and a solvent water molecule ( Fig. 1) due to the crystallographic inversion center located at the center of the cyclohexyl ring. The central cyclohexyl moiety of the dication adopts a chair conformation. The two trans-(4-pyridine)-CH 2 -NH 2 -moieties at the 1-and 4-positions of the central cyclohexyl ring occupy equatorial positions. The terminal pyridine ring is tilted by 27.98 (5) with respect to the mean plane through the central cyclohexyl moiety (r.m.s. deviation = 0.2379 Å ). The distance between the two terminal pyridine nitrogen atoms in the dication is 15.864 (2) Å . This is slightly shorter than the NÁ Á ÁN separation [15.970 (3) Å ] in the dication ligand of a one-dimensional zigzag-like Co II coordination polymer built up from alternate Co II ions and the dication of the title salt (Lee & Lee, 2010).

Figure 2
The two-dimensional undulating layer formed through intermolecular C-HÁ Á Á interactions (light-blue dashed lines) and N-HÁ Á ÁO/Cl and C-HÁ Á ÁCl hydrogen bonds (yellow dashed lines). H atoms not involved in intermolecular interactions have been omitted for clarity.

Figure 3
The three-dimensional supramolecular network formed through intermolecular N-HÁ Á ÁO hydrogen bonds (black dashed lines). Intermolecular C-HÁ Á Á interactions, and N-HÁ Á ÁO/Cl and C-HÁ Á ÁCl hydrogen bonds within the two-dimensional undulating layer are shown as light-blue and yellow dashed lines, respectively. H atoms not involved in intermolecular interactions have been omitted for clarity.
HÁ Á ÁCl hydrogen bonds (yellow dashed lines in Fig. 2) between the dications and the solvent water molecules/ chloride anions, respectively. Furthermore, neighboring undulating layers are connected through O-HÁ Á ÁN hydrogen bonds (black dashed lines in Fig. 3) between the solvent water molecules and the pyridine nitrogen atoms, forming a threedimensional supramolecular network. In addition, O-HÁ Á ÁCl hydrogen bonds ( Fig. 1 and Table 1) between the solvent water molecules and the chloride anions are also found in the crystal.

Synthesis and crystallization
2 M hydrochloric acid in ethanol was added to an ethanol solution of N,N-bis(pyridin-4-ylmethylene)cyclohexane-1,4diamine, synthesized according to a literature method (Huh & Lee, 2007), until pH = 4-5. The resulting mixture was left to evaporate slowly over several days, resulting in the formation of X-ray quality single crystals of the title salt.

N,N′-Bis(pyridin-4-ylmethyl)cyclohexane-1,4-diammonium dichloride dihydrate
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.