Crystal structure, Hirshfeld surface analysis and physicochemical characterization of bis[4-(dimethylamino)pyridinium] di-μ-chlorido-bis[dichloridomercurate(II)]

The two independent organic cations in the asymmetric unit of the chloromercurate(II) salt exhibit essentially the same features with almost planar pyridinium and dimethylamino groups. In the crystal, N—H⋯Cl and C—H⋯Cl hydrogen bonds as well as π–π and Cl⋯Cl interactions link the organic cations and inorganic anions into a three-dimensional network.

The title molecular salt, (C 7 H 11 N 2 ) 2 [Hg 2 Cl 6 ], crystallizes with two 4-(dimethylamino)pyridinium cations (A and B) and two half hexachloridodimercurate(II) anions in the asymmetric unit. The organic cations exhibit essentially the same features with an almost planar pyridyl ring (r.m.s. deviations of 0.0028 and 0.0109 Å ), which forms an inclined dihedral angle with the dimethyamino group [3.06 (1) and 1.61 (1) , respectively]. The dimethylamino groups in the two cations are planar, and the C-N bond lengths are shorter than that in 4-(dimethylamino)pyridine. In the crystal, mixed cation-anion layers lying parallel to the (010) plane are formed through N-HÁ Á ÁCl hydrogen bonds and adjacent layers are linked by C-HÁ Á ÁCl hydrogen bonds, forming a threedimensional network. The analyses of the calculated Hirshfeld surfaces confirm the relevance of the above intermolecular interactions, but also serve to further differentiate the weaker intermolecular interactions formed by the organic cations and inorganic anions, such asand ClÁ Á ÁCl interactions. The powder XRD data confirms the phase purity of the crystalline sample. Furthermore, the vibrational absorption bands were identified by IR spectroscopy and the optical properties were studied by using optical UV-visible absorption spectroscopy.

Chemical context
Hybrid organic-inorganic materials have been widely studied in recent years for their promising applications in different fields, including catalysis, magnetism and optics and for their luminescence properties (Clé ment et al., 1994;Rabu et al., 2001;Hu et al., 2003;Morris et al., 2008). However, owing to the confinement of the inorganic layers, the organic cations have to possess the right ionic bond and steric hindrance, as well as hydrogen bonds, to fit the coordination environment provided by the inorganic framework for stabilization of these organic-inorganic hybrid systems.
Hybrids based on mercury have been synthesized and characterized with simple, different techniques, thanks to their self-assembling character (Mitzi et al., 2001) and are very interesting both for fundamental physics exploration such as electronic confinement (Wei et al., 2015) or as low-dimensional magnetic systems (Fersi et al., 2015) and diversify the field of technological applications.
A number of chloromercurate(II) complexes have been shown to exhibit ferroelectric behaviour (Mitsui & Nakamura, 1990) and interest has focused on the mechanism of the ISSN 2056-9890 ferroelectric-paraelectric phase transition (White, 1963;Kö rfer et al., 1988;Jiang et al., 1995;Liesegang et al., 1995) for which structural information is crucial. In addition, the ability of the anions in this class of compounds to exhibit a wide range of geometry, stoichiometry and connectivity has long been known (Grdenic, 1965). This flexibility is a result of the large volume and spherical charge distribution of the Hg 2+ ion, which are a consequence of the filled 4f and 5d electron shells. Moreover, organic-inorganic materials with pyridine and its derivatives as template agents have led to the preparation of some materials with interesting physical properties (Aakerö y et al., 2000;Prince et al., 2003) and biological activities (Bossert et al., 1981;Wang et al., 1989).

Figure 1
The molecular structure of (I). Atomic displacement parameters for the non-H atoms are drawn at the 50% probability level. Unlabelled atoms are related to labelled ones by the symmetry operation Àx + 1, Ày, Àz + 2.

Vibrational study
The obtained FT-IR spectrum for the studied hexachloridodimercurate(II) salt is depicted in Fig. 7. Detailed assignment of all bands observed in the infrared spectrum of the 4-(dimethylamino)pyridinium cation in the title compound is based on the comparison with other compounds associated to the same cation (Koleva et al., 2008;Hu et al., 2012). In the region of high frequencies, the bands at 3243, 3130, 3100, 2959 cm À1 are due to the stretching vibrations of the N-H and C-H bonds. The band at 1646 cm À1 is assigned to the N-H bending mode. The bands at 1557 and 1445 cm À1 are attributed to the C C and C N stretching modes of the pyridine ring. The absorption band located at 1212 cm À1 corresponds to the     stacking interactions between the nearest aromatic organic cation neighbors into two types of organic columns (A or B). Table 1 Hydrogen-bond geometry (Å , ). range 1000 to 500 cm À1 are assigned to (C-C), (C-H) and (C-N) out-of-plane bending modes.

Optical properties and frontier molecular orbitals
Optical absorption (OA) measurement of the title compound was performed at ambient temperature in an ethanol solution (10 À4 M). As shown in Fig. 8, the OA spectrum exhibits two distinct absorption bands around 213 and 278 nm assigned to the !* absorption bands of the 4-(dimethylamino)pyridinium cations. Thus, the experimental band-gap energy obtained from the absorption edge wavelength is about 3.98 eV. This band-gap value indicates that the grown crystal exhibits semiconductor behavior (Rosencher & Vinter, 2002). The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), known as frontier orbitals, obtained with a B3LYP/6-311G+(d,p) [H, C, N, Cl]-LANL2DZ [Hg] level calculation are illustrated in Fig. 9. The HOMO is mainly delocalized at the pyridine ring system. After excitation, the charge is localized on the hexachloridodimercurate(II) moieties, as depicted in the LUMO. The calculated HOMO-LUMO energy gap (4.26 eV) is shifted from the experimental value, which may be attributed to solvent effects, compared to the gas-phase calculation. ClÁ Á ÁCl and HgÁ Á ÁCl interactions between hexachloridodimercurate(II) anions dispersed parallel to the a axis in the inorganic column.

Figure 7
Infrared spectrum of (I).

Figure 8
UV-vis spectrum of (I). The inset shows the experimental energy band gap obtained from the absorption edge wavelength.

Figure 9
HOMO-LUMO molecular orbitals showing the ground to excited state electronic transitions for (I).

Hirshfeld surface analysis
A Hirshfeld surface analysis (Spackman & Jayatilaka, 2009) and the associated two-dimensional fingerprint plots (McKinnon et al., 2007) were performed with Crystal-Explorer17 (Turner et al., 2017) to investigate the intermolecular interactions in the title compound. Fig. 10a illustrates the Hirshfeld surface mapped over d norm , which was plotted with a colour scale of À0.211 to 1.132 a.u. with a standard (high) surface resolution. The red spots highlight the interatomic contacts including the N-HÁ Á ÁCl and C-HÁ Á ÁCl hydrogen bonds. The shape-index of the Hirshfeld surface is a tool to visualize thestacking by the presence of adjacent red and blue triangles; if there are no adjacent red and/or blue triangles, then there are nointeractions. Fig.10b clearly suggests thatinteractions are present in the title hexachloridodimercurate(II) salt. Fig. 11a shows the two-dimensional fingerprint of all contacts contributing to the Hirshfeld surface. In Fig. 11b, with two symmetrical wings on the left and right sides illustrate the HÁ Á ÁCl/ClÁ Á ÁH interactions with a contribution of 49.5%. Fig. 11c illustrates the two-dimensional fingerprint plot of (d i , d e ) points related to HÁ Á ÁH contacts, which represent a 24.9% contribution.     Relative contribution (%) of various intermolecular interactions to the Hirshfeld surface area.

Synthesis and crystallization
The title compound was synthesized by dissolving 2 mmol (241 mg) of 4-dimethylaminopyridine 98% (Sigma-Aldrich) in an HCl 36-38% (Sigma-Aldrich) aqueous solution and 1 mmol (273 mg) of mercury(II) chloride HgCl 2 (Merck) in ethanol in a molar ratio of 2:1. The mixture was then stirred for 2 h. The resulting aqueous solution was filtered and then evaporated at room temperature, which finally led to the growth of parallelepipedic colourless crystals after one day.