Crystal structure of N,N′-bis(pyridin-3-ylmethyl)cyclohexane-1,4-diammonium dichloride

The title salt, C18H26N4 2+·2Cl−, is located about a crystallographic inversion centre at the centre of the cyclohexyl ring, which adopts a chair conformation. In the crystal, N+(C)—H⋯Cl− hydrogen bonds and π–π stacking interactions lead to the formation of a three-dimensional supramolecular network.


Chemical context
Several dipyridyl-type ligands with or without a central section between the terminal pyridine rings have contributed greatly to the development of metal-organic coordination polymers with intriguing topologies or potential applications (Silva et al., 2015;Furukawa et al., 2014;Robin & Fromm, 2006;Robson, 2008;Leong & Vittal, 2011). Our group has also tried to prepare extended dipyridyl-type ligands with a bulky central moiety for the construction of versatile coordination polymers. Recently, we prepared the dipyridyl-type ligand containing 4-pyridine terminal groups and a cyclohexyl ring as a bulky central moiety, namely N,N-bis(pyridin-4-ylmethyl)cyclohexane-1,4-diamine, and reported the crystal structure of its chloride salt (Moon et al., 2016). As an extension of our research, we have prepared a dipyridyl-type ligand with central cyclohexyl ring and 3-pyridine terminal groups, namely N,N-bis(pyridin-3-ylmethyl)cyclohexane-1,4-diamine, synthesized by a condensation reaction between 1,4-cyclohexanediamine and 3-pyridinecarboxaldehyde according to the literature procedure (Huh & Lee, 2007). Herein we report on crystal structure of the title salt obtained by the protonation of both amine groups in this molecule. Fig. 1 shows the molecular structure of the title salt, which lies about an inversion centre located at the centre of the cyclohexyl ring. Therefore, the asymmetric unit comprises one half ISSN 2056-9890 of the N,N-bis(pyridin-3-ylmethyl)cyclohexane-1,4-diammonium dication and a chloride anion. In the dication, the central cyclohexyl ring displays a chair conformation and the two trans-(4-pyridine)-CH 2 -NH 2 -moieties occupy equatorial sites at the 1-and 4-positions of the central cyclohexyl ring. The terminal pyridine ring is tilted by 53.72 (6) with respect to the mean plane of the cyclohexyl ring (r.m.s. deviation = 0.2413 Å ). This tilting angle is larger than that [27.98 (5) ] of the similar dication with 4-pyridine rings as the terminal groups (Moon et al., 2016).

Supramolecular features
In the crystal, N + -HÁ Á ÁCl À hydrogen bonds, Table 1 (yellow dashed lines in Figs. 2 and 3), between the dications and the chloride anions lead to the formation of chains along the b axis. Adjacent chains are additionally connected through intermolecularstacking interactions [centroid-to-centroid distance = 3.8197 (8) Å ] between the pyridine rings (red dashed lines in Figs. 2 and 3), resulting in the formation of a sheet extending parallel to the ab plane. These sheets are linked by weak C-HÁ Á ÁCl À hydrogen bonds, Table 1 (black dashed lines in Fig. 3), between the dications and the chloride anions, forming a three-dimensional supramolecular network. A view of the molecular structure of the title salt, showing the atomnumbering scheme. Displacement ellipsoids are drawn at the 50% probability level. H atoms are shown as small spheres of arbitrary radius and yellow dashed lines represent the intermolecular N + -HÁ Á ÁCl À hydrogen bonds. [Symmetry code: (i) Àx + 1, Ày + 1, Àz + 1.] Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
The two-dimensional sheet of the title salt formed through intermolecular N + -HÁ Á ÁCl À hydrogen bonds (yellow dashed lines) between the dications and the chloride anions andstacking interactions (red dashed lines) between the pyridine rings of dications. H atoms not involved in intermolecular interactions have been omitted for clarity.

Figure 3
The three-dimensional supramolecular network of the title salt formed through intermolecular C-HÁ Á ÁCl À hydrogen bonds (black dashed lines) between the two-dimensional sheets constructed by intermolecular N + -HÁ Á ÁCl À hydrogen bonds (yellow dashed lines) andstacking interactions (red dashed lines). H atoms not involved in intermolecular interactions have been omitted for clarity.
complex is six-coordinated by two O atoms of one nitrate anion and four N atoms of four dipyridyl-type dication ligands to form a distorted octahedral geometry.

Synthesis and crystallization
N,N-bis(pyridin-3-ylmethylene)cyclohexane-1,4-diamine, prepared according to a literature method (Huh & Lee, 2007), was dissolved in ethanol, and then the pH was adjusted to 4-5 with 2 M hydrochloric acid. The resultant 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-3-ylmethyl)cyclohexane-1,4-diammonium dichloride
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 )