Structural characterization and Hirshfeld surface analysis of a CoII complex with imidazo[1,2-a]pyridine

The complex [CoL 2Cl2] (L = imidazo[1,2-a]pyridine) exhibits a supramolecular-layered assembly through π–π stacking interactions. The overall intermolecular interactions involved in the structure have been quantified and fully described by Hirshfeld surface analysis.

Inspired by the manifold potential applications of imidazo[1,2-a]pyridine, we focused our attention on its ISSN 2056-9890 coordination behavior towards metal ions and to the structural features of the resulting complexes. Herein, the crystal and molecular structure of a new Co II complex with imidazo-[1,2-a]pyridine is described, along with an investigation of the intermolecular interactions via Hirshfeld surface analysis.

Structural commentary
The molecular structure of the title complex is shown in Fig. 1. The Co II ion is located on a twofold axis, so that half of the complex is generated by symmetry. The metal center is coordinated to the nitrogen atoms of two imidazopyridine ligands and to two chlorine ions, and shows a tetrahedral geometry with angles ranging from 107.70 (5) to 112.44 (5) . Selected geometric parameters around Co II are reported in Table 1. The imidazopyridine moiety is planar, with a dihedral angle between the rings of 2.47 (3) . In the imidazopyridine moiety, atoms C6 and C4 show the largest deviations in opposite directions [C6: +0.034 (1) and N1: À0.037 (1)] from the least-squares mean plane through the atoms N1/C6/C7/N2/ C1-C5.

Supramolecular features
The title structure exhibits intermolecular C-HÁ Á ÁCl andstacking interactions; the details are included in Tables 2 and  3, respectively. It is convenient to consider the 'substructures' generated by each interaction individually, and then combine these substructures to build up the supramolecular assembly.
The first substructure is formed considering the pyridine ring carbon atom C5 in a general position, which acts as donor to the Cl1 atom at (Àx, Ày, 1 À z). This C5-H5Á Á ÁCl1 interaction and its centrosymmetric analogue generate an R 2 2 (18) dimeric ring (M) centered at (0, 0, 1/2) (Fig. 2). A second substructure is formed via pairs of symmetry-related C7-H7Á Á ÁCl1(x, À1 + y, z) interactions, which generate a dimeric R 2 2 (10) ring (N) (Fig. 2). The propagation of these dimers produces two infinite chains, the first running parallel to the (101) plane and the second running parallel to the [010] direction. The interconnection of the two chains leads to the generation of another tetrameric R 2 4 (14) ring motif (P ORTEP view with atom-numbering scheme of the title complex with displacement ellipsoids drawn at the 30% probability level. The unlabeled counterpart is generated by the symmetry operation Àx + 1 2 , y, Àz + 3 2 .  Symmetry codes: (ii) Àx; Ày; Àz þ 1; (iii) x; y À 1; z.

Figure 2
Formation of a two-dimensional supramolecular network generated through self-complementary C-HÁ Á ÁCl interactions.
Another substructure can be described considering that the molecules, because of their self-complementarity nature, are juxtaposed throughstacking interactions (Seth et al., 2011aManna et al., 2013Manna et al., , 2014a. The molecular packing of the complex is such that thestacking interactions between the pyridine rings, as well as between the imidazo rings, are optimized. The pyridine rings of the molecules at (x, y, z) and (Àx + 1, Ày, Àz + 1) are strictly parallel, with an interplanar spacing of 3.4671 (9) Å and a ring-centroid separation of 3.5293 (16) Å , corresponding to a ring offset of 0.659 Å . In addition, the imidazo rings at (x, y, z) and (Àx, Ày, Àz + 1) are juxtaposed through face-to-face -stacking with an inter-centroid separation of 3.6414 (16) Å . Moreover, the imidazo and pyridine rings of the parent molecules are also involved into multi -stacking interactions with each other. In particular, the interplanar spacing between the imidazo ring in a general position and the pyridine rings at (Àx, Ày, Àz + 1) and (Àx + 1, Ày, Àz + 1) are of 3.5303 (9) and 3.4625 (9) Å , respectively, while the relative ring-centroid separations are 3.9583 (16) and 3.8371 (16) Å . Thesestacking interactions result in a two-dimensional supramolecular layered assembly parallel to the (010) plane (Fig. 3).

Hirshfeld surface analysis
Molecular Hirshfeld surfaces (Spackman & McKinnon, 2002) in the crystal structure are constructed considering the electron distribution calculated as the sum of spherical atom electron densities (Spackman & Byrom, 1997;McKinnon et al., 1998). The normalized contact distance (d norm ) based on both d e and d i , and the van der Waals (vdw) radii of the atom, given by the equation enable the identification of the regions of particular importance to intermolecular interactions (McKinnon et al., 2007). The combination of d e and d i in the form of a two-dimensional fingerprint plot (Rohl et al., 2008) provides a summary of the intermolecular contacts in the crystal (Spackman & McKinnon, 2002). The Hirshfeld surfaces are mapped with d norm , and the two-dimensional fingerprint plots presented in this work were generated using CrystalExplorer 3.1 (Wolff et al., 2012). The pattern of the intermolecular interactions of the solidstate structure of the title complex prompted us to explore and quantify the contribution of the non-covalent interactions in the crystal packing, as well as the importance of the C-HÁ Á ÁCl bonding in directing the organization of the extended supramolecular network (Seth et al., 2011a,b, Manna et al., 602 Saikat Kumar Seth [CoCl 2 (C 7 H 6 N 2 ) 2 ] Acta Cryst. (2018). E74, 600-606 weak interactions in crystals Monomeric units linked through multistacking interactions leading to the formation of a supramolecular layered assembly. Color codes: the green and yellow dotted lines denotestacking interactions between two pyridine rings and two imidazo rings, respectively, whereasstacking interactions between pyridine and imidazo rings are represented by pink dotted lines. Table 3 Geometrical parameters (Å , ) forstacking.
(a) Cg1 and Cg2 are the centroids of the (N1/C1/N2/C6/C7) and (N2/C1-C5) rings, respectively; (b) centroid-centroid distance between ring i and ring j; (c) vertical distance from ring centroid i to ring j; (d) vertical distance from ring centroid j to ring i; (e) dihedral angle between the first ring mean plane and the second ring mean plane of the partner molecule; (f) angle between the centroid of the first ring and the second ring; (g) angle between the centroid of the first ring and the normal to the mean plane of the second ring of the partner molecule.  (16) 3.4671 (9) 3.4671 (9) 0.0 10.77 10.77 0.659 Symmetry codes: (ii) Àx, Ày, Àz + 1; (iv) Àx + 1, Ày, Àz + 1.

Figure 4
Hirshfeld surfaces of the title complex mapped with (a) d norm , (b) d e , (c) shape-index and (d) curvedness.
2012; Seth, 2013;Mitra et al., 2014). In this present investigation, the contacts responsible for building the supramolecular assembly were evaluated with respect to their contribution to the overall stability of the crystal structure. In this context, the Hirshfeld surface analysis (Spackman & McKinnon, 2002;Seth et al., 2011a,b,c,d;Mitra et al., 2013) of the title complex was performed and the results are illustrated in Fig. 4. The surfaces represented were mapped over d norm , d e , shape-index and curvedness in the ranges À0.0620 to 0.9660 Å , 1.0626 to 2.4714 Å , À1.0000 to 1.0000 Å and À4.0000 to 0.4000 Å , respectively. The information regarding the intermolecular interactions summarized in Tables 2 and 3 are visible as spots on the Hirshfeld surfaces (Fig. 4). For instance, the distinct circular depressions (red spots) on the d norm surface (Fig. 4a) are due to the C-HÁ Á ÁCl contacts, whereas other visible spots are due to HÁ Á ÁH contacts. From the Hirshfeld surfaces, it is also evident that the molecules are related to one another by stacking interactions, as can be inferred from inspection of the adjacent red and blue triangles (highlighted by yellow circles) on the shape-index surface (Fig. 4c). Indeed, the pattern of red and blue triangles in the same region of the shape-index surface is characteristic ofstacking interactions; the blue triangles represent convex regions resulting from the presence of ring carbon atoms of the molecule inside the surface, while the red triangles represent concave regions caused by carbon atoms of the -stacked molecule above it. The presence ofstacking is also evident in the flat region toward the bottom of both sides of the molecules and is clearly visible on the curvedness surface (Fig. 4d): the shape of the blue outline on the curvedness surface unambiguously delineates the contacting patches of the molecules. On the d e surface, this feature appears as a relatively flat green region where the contact distances are similar (Fig. 4b).
The intermolecular interactions present in the structure are also visible on the two-dimensional fingerprint plot (Rohl et al., 2008;Samanta et al., 2014;Seth, 2014a,b,c), which can be decomposed to quantify the individual contributions of each intermolecular interaction involved in the structure (Manna et al., 2014b). These complementary regions are visible in the fingerprint, where one molecule acts as donor (d e > d i ) and the other as an acceptor (d i > d e ). Table 4    of contributions for a variety of contacts in the crystal structure of the title compound. The C-HÁ Á ÁCl interactions appear as two distinct spikes in the fingerprint plot (Fig. 5) of the title complex, where ClÁ Á ÁH interactions have a larger contribution (18.4%) than their HÁ Á ÁCl counterparts (11.6%). Thus, the sum of ClÁ Á ÁH/HÁ Á ÁCl interactions comprises 30.0% of the total Hirshfeld surface area of the molecule (Table 4). The ClÁ Á ÁH/HÁ Á ÁCl interactions represented by the spikes in the bottom right and left region (d e + d i ' 2.77 Å ) indicate that the hydrogen atoms from the ligand moiety are in contact with the metal-coordinated Cl atoms to build the two-dimensional supramolecular framework. The spoon-like tips in the region (d e + d i ' 3.37 Å ) of the fingerprint plot (Fig. 5) represent a significant NÁ Á ÁH/HÁ Á ÁN contribution, covering 4.1% of the total Hirshfeld surface of the molecules. The forceps-like tips in the region (d e + d i ' 3.12 Å ) of the fingerprint plot (Fig. 5) represent the CÁ Á ÁH/HÁ Á ÁC contacts where the CÁ Á ÁH counterpart shows a larger contribution (7.6%) than the HÁ Á ÁC counterpart (4.5%). Overall, the CÁ Á ÁH/HÁ Á ÁC interactions account for 12.1% of the total Hirshfeld surface of the molecules (Table 4), and the carbon atoms of the imidazopyridine moiety mainly act as donors in building the molecular assembly. The scattered points in the breakdown of the fingerprint plot show that thestacking interactions comprise 7.9% of the total Hirshfeld surface of the molecule (Table 5) displayed as a region of blue/green color on the diagonal at around d e ' d i ' 1.743 Å . Another contribution comes from HÁ Á ÁH contacts (38.4%) represented by the scattered points in the fingerprint plots, and spread up only to d i = d e = 1.092 Å (Fig. 5).

Database survey
A search in the Cambridge Structural Database (Version 5.38, update May 2017; Groom et al., 2016) for structures of the general formula [ML 2 X 2 ], where M is any transition metal, L is the ligand imidazo[1,2-a]pyridine, and X any halogen, yielded no results. However, two related complexes exist, with ruthenium and tin, respectively: (i) dichloro-[2,2 0 -(pyridine-2,6-diyl)bis(imidazo[1,2-a]pyridine)]triphenylphosphineruthenium(II) (GULNEI; Li et al., 2015); (ii) dibromo-bis-(imidazo[1,2-a]pyridine)dimethyltin (NODREF; Agrawal et al., 2014). In both cases, the presence of the halogen atoms is relevant to the stabilization of the crystal structure. In the case of the ruthenium compound, the complex molecules are linked into discrete supramolecular dimers through pairs of C-H(imidazo)Á Á ÁCl interactions. On the other hand, the tin complex forms undulating sheets parallel to the (100) plane by means of C-H(pyridine)Á Á ÁBr interactions in which both the Br ions and the ligands of one complex act as acceptor and donor, respectively.

Synthesis and crystallization
The title complex was prepared by simple hydrothermal reaction. CoCl 2 Á6H 2 O (2.0 mmol, 0.476 g) was dissolved in water (20 ml) yielding a clear pink solution. A hot watermethanol (1:1) solution (20 ml) of imidazo[1,2-a]pyridine (1.0 mmol, 0.118 g) was added dropwise to the above solution under continuous stirring. The solution mixture thus obtained was further heated at 343 K for 2 h and then kept for crystallization at room temperature (303 K). The resulting solution was allowed to evaporate slowly at room temperature for several weeks, yielding testable dark-pink crystals, which were collected by filtration, washed with water and dried in air.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 6. The hydrogen atoms were located in the difference-Fourier map and refined as riding atoms, with C-H = 0.93 Å and U iso (H) = 1.2U eq (C).

Dichloridobis(imidazo[1,2-a]pyridine-κN 1 )cobalt(II)
Crystal data [CoCl 2 (C 7  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.  (13)