Bis(2-methylpyridinium) tetrachloridocuprate(II): synthesis, structure and Hirshfeld surface analysis

The structure of bis(2-methylpyridinium) tetrachlorocuprate(II) is characterized by layers formed by [C6H8N]+ and [CuCl4]2– connected through N—H⋯Cl and C—H(phenyl)⋯Cl interactions. The layers are further connected by C—H(methyl)⋯Cl interactions.


Chemical context
Supramolecular organic and inorganic chemistry have been studied both from the fundamental as well as the application point of view, which is evident from the literature (Ziach et al., 2018;Thorat et al., 2013;Burslem et al., 2016). With the surge in the number of compounds reported, potential applications of supramolecular inorganic materials in energy storage, separation, catalysis, sensors, molecular magnets, optoelectronic materials, etc., have attracted greater attention in recent years (Mueller et al., 2006;Wan et al., 2006;Fé rey et al., 2003;James, 2003;Eddaoudi et al., 2002;Ruben et al., 2005, Kitagawa et al., 2004, Stavila et al., 2014. Because of the divergent combination of ligands and metal salts, an enormous number of structural architectures with different sizes and shapes could be constructed (Moulton & Zaworotko, 2001). The special characteristics and features such as ease of synthesis of the material, geometrically well-defined structures, exceptional tunability, post-synthetic modification, along with robustness of the material resulting from strong directional bonding, produce new opportunities and offer a unique platform amenable to the synthesis of more and more functional solids. For example, Adams et al. (2005) reported the synthesis of coordination compounds using a new synthetic route involving a thermal dehydrochlorination reaction in crystals of a pyridinium chlorometallate bicomponent system, i.e., anionic metal complexes and organic cations.
As part of ongoing studies in our group (PrakashaReddy & Pedireddi, 2007;Reddy et al., 2014), the synthesis of coordination complexes using pyridine ligands has been reported. Hence, we further extended our studies to utilize the pyridinium ligand and to study in situ the single-crystal-to-singlecrystal transition (SCSCT) to investigate the reaction mechanism. In our endeavours to synthesize new functional solids, using a transition-metal anion and a pyridinium cation, we have chosen the CuCl 2 and 2-methylpyridinium salt complex. Herein, we report the synthesis and crystal structure ISSN 2056-9890 of a bis(2-methylpyridinium) tetrachlorocuprate coordination complex.

Structural commentary
The title complex crystallizes in the monoclinic space group I2/c. Since the Cu 2+ cation occupies a special position, the asymmetric unit consists of a 2-methylpyridinium cation, [2-Me(Py)H] + , and half of a tetrachlorocuprate(II) anion, [CuCl 4 ] 2-. The molecular structure of the complex along with the atom-labelling scheme is shown in Fig. 1. Each copper center is four-coordinated by chlorine anions and adopts a distorted tetrahedral geometry. Structural analysis shows that the Cl-Cu-Cl angles vary from 98.55 (2) to 137.4 (3) with four angles smaller and two larger than the standard tetrahedral angle. A plausible reason for a larger deviation from the standard 109.5 might be due to the N-HÁ Á ÁCl and C-HÁ Á ÁCl interactions. A similar marked deviation from the standard tetrahedral angle has been previously observed by other research groups (Wyrzykowski et al., 2011;Jasrotia et al., 2018). The Cu-Cl bond lengths [Cu1-Cl1 = 2.250 (1) Å , Cu1-Cl2 = 2.249 (1) Å ] agree well with those reported for other structures (Marsh et al., 1982;Dodds et al., 2018;Molano et al., 2020;Reddy, 2020). The intramolecular C ar -C ar bond lengths in the [2-Me(Py)H] + fall in the range 1.370 (3)-1.395 (3) Å . The N1-C2 and N1-C6 bond lengths are 1.350 (2) and 1.346 (2) Å , respectively.

Supramolecular features and Hirshfeld surface analysis
In the crystal, complex molecules related by the twofold rotation axis are connected by pairs of N-HÁ Á ÁCl and C ar -HÁ Á ÁCl interactions through a protonated N and an aromatic hydrogen attached to the carbon atom with the chloride ligand bonded to copper, forming a monomeric unit. These units interact with adjacent ones through C ar -HÁ Á ÁCl hydrogen bonding (Table 1 Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
The N-HÁ Á ÁCl and C-HÁ Á ÁCl interactions between cations and anions in the crystal structure of the title compound.

Figure 3
The crystal packing of the title compound viewed along the b axis with intermolecular contacts shown as dashed lines.

Figure 1
The molecular structure of the title compound, showing the atom labelling and displacement ellipsoids drawn at the 50% probability level. Hydrogen atoms are shown as small spheres of arbitrary size. [Symmetry code: , 2011;Jasrotia et al., 2018). The supramolecular structure is further stabilized by C methyl -HÁ Á ÁCl interactions involving hydrogens of the methyl group and chlorides bonded to copper, generating layers along the crystallographic b axis (Fig. 3).
To further investigate the intermolecular interactions present in the title compound, a Hirshfeld surface analysis was performed and the two-dimensional fingerprint plots were generated with Crystal Explorer17 (Turner et al., 2017). The Hirshfeld surface mapped over d norm and corresponding colours representing various interactions are shown in Fig. 4. The red points on the surface correspond to the N-HÁ Á ÁCl and C-HÁ Á ÁCl interactions. The two-dimensional fingerprint plots (McKinnon et al., 2007) are shown in Fig. 5. On the Hirshfeld surface, the largest contribution (53.1%) comes from the weak van der Waals HÁ Á ÁH contacts. The interaction of d norm on the two-dimensional fingerprint plot shows two spikes; each one corresponds to HÁ Á ÁH (39%) and HÁ Á ÁCl/ ClÁ Á ÁH (32.5%) respectively. The HÁ Á ÁCl interaction highlights the hydrogen bond between adjacent moieties in the crystal structure. The CÁ Á ÁH/HÁ Á ÁC (16.5%) interactions appear as two shoulders. These interactions play a crucial role in the overall stabilization of the crystal packing. (CCDC refcode CMPYCU01; Marsh et al., 1982) and [aqua-dichloro-bis(2methylpyridine)Cu(II)] (CCDC refcode BIJWUM; Marsh et al., 1982) and a very recently published dichloridomethanolbis(2-methylpyridine)Cu(II) complex (Reddy, 2020). All of these structures display three-dimensional supramolecular networks stabilized by C-HÁ Á ÁCl and O-HÁ Á ÁCl interactions.

Synthesis and crystallization
Both 2-methylpyridine and anhydrous copper(II) chloride obtained from Aldrich were used for the reaction. Anhydrous copper(II) chloride (0.495 g, 0.005 mol) was dissolved in 10 ml of distilled water. To this solution, 2-methylpyridine (0.93 g, 0.01 mol) was added followed by addition of few drops of HCl (36%) and the resulting mixture was stirred for $30 min. at room temperature. The solution was then allowed to stand at room temperature for a few hours before being filtered and left at room temperature for crystallization. Block-shaped, pale-yellow-coloured crystals were obtained after 36 h.

Bis(2-methylpyridinium) tetrachloridocuprate(II)
Crystal data (C 6  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.