Bis(imidazo[1,2-a]pyridin-1-ium) tetrachloridocuprate(II) dihydrate

Copper is known for its variable coordination geometry adopted for different ligands. Here we report the crystal structure of a new imidazo[1,2-a]pyridin-1-ium tetrachloridocuprate(II) salt with a distorted tetrahedral geometry for the copper atom.

In the title salt, (C 7 H 7 N 2 ) 2 [CuCl 4 ]Á2H 2 O, the Cu 2+ cation is coordinated by four Cl atoms and adopts a distorted tetrahedral geometry. Two molecules of imidazo[1,2-a]pyridine are protonated ensuring electrical neutrality. O-HÁ Á ÁCl and N-HÁ Á ÁO hydrogen bonds link the organic and the inorganic moieties, leading to a self-organized hydrated hybrid structure.

Chemical context
Copper halides have applications in biology as antifungal and anticancer agents (Creaven et al., 2010;Santini et al., 2014) and are also good precursors for photovoltaic cells because of their optoelectronic and magnetic properties (Levitsky et al., 2004;Ahmadi et al., 2013;Al-Far & Ali, 2009). For this reason, we have focused our research on copper-based hybrid materials using diverse organic moieties to balance the halide copper inorganic anions. We report in this paper the synthesis and structure determination using single crystal X-ray diffraction data of a tetrahedral tetrachloridocuprate(II) anion with imidazo[1,2-a]pyridin-1-ium organic cations and two lattice water molecules.
When coordinated by halide anions, copper can adopt several coordination geometries including tetrahedral, squarepyramidal, square-planar and square-bipyramidal (Bhattacharya et al., 2004;Yuan et al., 2004). A four-coordinate geometry is generally intermediate between square-planar and regular-tetrahedral, as reported by Al-Far & Ali (2009). In our case and according to the angular values of the copperchlorine bonds, summarized in Table 1, the tetrahedral copper coordination seems to be slightly distorted. These distortions are a consequence of the lower molecular symmetry.
The two water molecules are located approximately in a common plane defined by the organic cations, directing their hydrogen atoms towards the anionic group [CuCl 4 ] 2À and leaving the oxygen free-electron pairs available for a hydrogen-bonding interaction with the protonated nitrogen site of the imidazo[1,2-a]pyridinum cation. In the anionic subnetwork, every [CuCl 4 ] 2À anion is linked to two water molecules by hydrogen bonds via the Cl2 vertices, as shown in Fig. 2.
In spite of the single protonation of the organic molecule on the aromatic nitrogen site, every cation is linked to two water molecules through bifurcated hydrogen-bonding interactions along [010], as shown in Fig. 3. The organic cations are organized along (010), forming sheets parallel to the ab plane. A projection of the structural packing along the c axis, Fig. 4, reveals alternating empty elliptical channels delimited by the organic cations and inorganic tetrahedra. The long and short dimensions of the elliptical sections are estimated to be, respectively, 6.1 (1) and 2.1 (1) Å for the largest ones and 4.3 (1) and 1.4 (1) Å for the narrowest. These voids are able to lodge several small solvent molecules.  (7) Symmetry code: (i) Àx þ 1; y; Àz þ 1 2 .

Figure 2
The [CuCl 4 ] 2À environment with hydrogen bonds shown as blue dashed lines. Displacement ellipsoids are displayed at the 50% probability level.

Figure 4
Crystal packing along the c axis showing empty tunnels able to lodge small organic solvent molecules. Displacement ellipsoids are displayed at the 50% probability level.
The water molecules play a crucial role in the crystalpacking cohesion. Every water molecule is linked to one [CuCl 4 ] 2À tetrahedron through O-HÁ Á ÁCl hydrogen bonds (Table 2) and to two organic molecules through O-HÁ Á ÁN hydrogen bonds, as shown in Fig. 5. The expected structural self-organization generally present in hybrid inorganicorganic compounds can also be found in the structure of the title salt. The alternating stacking of organic and inorganic sheets observed along the c axis ( Fig. 6) could possibly lead to luminescence properties.

Supramolecular features
The lowering of the symmetry of the copper coordination could also be due to halide-halide and intra-and intermolecular hydrogen-bonding interactions; these interactions are closely related to the shape and the size of the counter-cations (Bouacida et al., 2013;Parent et al., 2007;Haddad et al., 2006;Marzotto et al., 2001;Choi et al., 2002;Awwadi et al., 2007). Non-covalent interactions such as hydrogen-bonding interactions andstacking interactions represent the most important linkers in this kind of material. Moreover, these interactions are able to delimit not only the architecture, but also impact on the properties of metal-halide materials. The organic cations are linked to the water molecule through N1-H1AÁ Á ÁOW hydrogen bonds (Table 2) and are connected through face-to-facestacking [Cg1Á Á ÁCg2( 3 2 À x, 1 2 À y, 1 À z) = 3.968 (3) Å where Cg1 and Cg2 are the centroids of the N1/N3/C1-C3 and N2/C3-C7 rings, respectively]. The crystal packing can be described by alternating stacks of anions and cationic chains with the organic layers arranged parallel to the anionic stacks. The environment around the water molecule. Hydrogen bonds are indicated by blue dashed lines. Displacement ellipsoids are displayed at the 50% probability level. [Symmetry codes: (i) 1 À x, y, 1 2 À z; (iii) 1 2 À x, À 1 2 + y, 1 2 À z.]

Figure 6
View of the packing showing the alternating stacking of the organic and inorganic layers connected through hydrogen bonds. The face-to-facestacking between parallel organic molecules is noteworthy with a centroid-centroid distance of 3.968 (3) Å . Displacement ellipsoids are displayed at the 50% probability level. [Symmetry code: (iii) 1 2 À x, À 1 2 + y, 1 2 À z.] them. To the best of our knowledge, this work is the first chemical and crystallographic identification of tetrachloridocuprate(II) combined with imidazo[1,2-a]-pyridyn-1ium.

Synthesis and crystallization
The title salt was prepared by the reaction of imidazo[1,2-a]pyridine and Cu(NO 3 ) 2 Á2H 2 O (molar ratio 1:1) in an equal volume of water and ethanol (10 ml) mixed with 2 ml of hydrochloric acid (37%). The solution was stirred for 1 h at 333 K. Prismatic yellow crystals suitable for X-ray diffraction were grown in one week by slow evaporation at room temperature.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms were positioned geometrically and treated as riding on the parent atom with C-H = 0.93 Å and N-H = 0.86 Å . For HW1 and HW2, the restraints DFIX and DANG were used to stabilize the water molecule.

Refinement
Refinement on F 2 Least-squares matrix: full R[F 2 > 2σ(F 2 )] = 0.038 wR(F 2 ) = 0.108 S = 1.04 2127 reflections 120 parameters 3 restraints Hydrogen site location: mixed H atoms treated by a mixture of independent and constrained refinement w = 1/[σ 2 (F o 2 ) + (0.0535P) 2 + 3.2027P] where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.78 e Å −3 Δρ min = −0.49 e Å −3 sup-2 Acta Cryst. (2017). E73, 196-199 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. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.  (17)