Crystal structure of an HgII coordination polymer with an unsymmetrical dipyridyl ligand: catena-poly[[[dichloridomercury(II)]-μ-N-(pyridin-4-ylmethyl)pyridin-3-amine-κ2 N:N′] chloroform hemisolvate]

In the title compound, each HgII ion is coordinated by two pyridine N atoms from two symmetry-related unsymmetrical dipyridyl ligands and two chloride anions in a highly distorted tetrahedral geometry. Each unsymmetrical dipyridyl ligand links two HgII ions into polymeric zigzag chains. In the crystal, the chains are linked into a three-dimensional supramolecular network by intermolecular N/C—H⋯Cl hydrogen bonds and weak C—H⋯π interactions. Weak C—Cl⋯π interactions and Cl⋯Cl contacts between the network and the solvent chloroform molecules are also observed.

The asymmetric unit of the title compound, {[HgLCl 2 ]Á0.5CHCl 3 } n (L = N-(pyridin-4-ylmethyl)pyridin-3-amine, C 11 H 11 N 3 ), contains one Hg II ion, one bridging L ligand, two chloride ligands and a chloroform solvent molecule with half-occupancy that is disordered about a crystallographic twofold rotation axis. Each Hg II ion is coordinated by two pyridine N atoms from two symmetryrelated L ligands and two chloride anions in a highly distorted tetrahedral geometry with bond angles falling in the range 99.05 (17)-142.96 (7) . Each L ligand bridges two Hg II ions, forming polymeric zigzag chains propagating in [010]. In the crystal, the chains are linked by intermolecular N/C-HÁ Á ÁCl hydrogen bonds together with weak C-HÁ Á Á interactions, resulting in the formation of a three-dimensional supramolecular network, which is further stabilized by C-ClÁ Á Á interactions between the solvent chloroform molecules and the pyridine rings of L [chloride-to-centroid distances = 3.442 (11) and 3.626 (13) Å ]. In addition, weak ClÁ Á ÁCl contacts [3.320 (5) Å ] between the chloroform solvent molecules and the coordinating chloride anions are also observed.

Chemical context
A variety of coordination polymers have been explored extensively over the last two decades because of their fascinating architectures and their useful applications in materials chemistry (Silva et al., 2015;Furukawa et al., 2014;Robson, 2008;Leong & Vittal, 2011). In this area of research, symmetrical dipyridyl ligands composed of two terminal pyridines with same substituted nitrogen positions have been used mainly for the design and construction of the coordination polymers. By contrast, investigations based on unsymmetrical dipyridyl ligands, with the nitrogen atoms in different positions on each of the two terminal pyridines, are still rare (Uemura et al., 2008;Khlobystov et al., 2003). Recently, our group and that of Gao have already reported Ag I coordination polymers with some unsymmetrical dipyridyl ligands such as N-(pyridine-3-ylmethyl)pyridine-2-amine (Lee et al., 2013;Zhang et al., 2013), N-(pyridine-2-ylmethyl)pyridine-3-amine (Ju et al., 2014;Zhang et al., 2013) and N-(pyridine-4-ylmethyl)pyridine-3-amine (Lee et al., 2015;Moon et al., 2015;Zhang et al., 2013). As a part of our ongoing efforts to construct coordination polymers with such unsymmetrical dipyridyl ligands, we prepared the title compound obtained by the reaction of mercury(II) chloride with an unsymmetrical dipyridyl ligand, namely N-(pyridine-4ylmethyl)pyridine-3-amine, synthesized according to a literature procedure (Lee et al., 2013). Herein, we report the crystal structure of the title compound.

Structural commentary
The asymmetric unit of the title compound, {[HgLCl 2 ]Á0.5CHCl 3 } n , L = N-(pyridine-4-ylmethyl)pyridine-3-amine, C 11 H 11 N 3 , comprises one Hg II ion, one L ligand, two chloride anions and one half-molecule of chloroform. The solvent molecule is disordered over two orientations of equal occupancy about the crystallographic twofold rotation axis. As shown in Fig. 1, the coordination geometry of each Hg II ion is highly distorted tetrahedral with two coordination sites being occupied by two pyridine N atoms from two symmetry-related L ligands. The geometry of the Hg II ion is completed by the coordination of two chloride ions. The tetrahedral angles around the Hg II ion fall in the range of 99.05 (17)-142.96 (7) ( Table 1).
Each L ligand bridges two Hg II ions into an infinite zigzag chain propagating along the b axis (Fig. 2). The separation between the Hg II ions through a L ligand in the chain is 8.1033 (6) Å . In the L ligand, the C py -N-C-C py torsion angle is À70.9 (7) while the dihedral angle between two terminal pyridine ring planes is 85.0 (2) . The conformation of the L ligand, along with the the N py -Hg-N py coordination angle [99.05 (17) ], may induce the zigzag topology of the chain.

Synthesis and crystallization
The L ligand was synthesized according to a literature method (Lee et al., 2013). X-ray-quality single crystals of the title compound were obtained by slow diffusion of a methanol solution of HgCl 2 into a chloroform solution of the L ligand.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. A reflection affected by the beamstop was omitted from the final refinement. The chloroform molecule is disordered over two sets of sites about a twofold rotation axis with equal occupancy. The C-Cl bond lengths were restrained using the DFIX instructions in SHELXL2014/7 (Sheldrick, 2015). All H atoms were positioned geometrically with d(C-H) = 0.93 Å for Csp 2 -H, 0.97 Å for methylene C-H, 0.98 Å for methine C-H, and 0.86 Å for amine N-H, and were refined as riding with U iso (H) = 1.2U eq (C,N). Computer programs: APEX2 and SAINT (Bruker, 2014), SHELXS97 and SHELXTL (Sheldrick, 2008), SHELXL2014 (Sheldrick, 2015) and DIAMOND (Brandenburg, 2010).

catena-Poly[[[dichloridomercury(II)]-µ-N-(pyridin-4-ylmethyl)pyridin-3-amine-κ 2 N:N′] chloroform hemisolvate]
Crystal data [HgCl 2 (C 11  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 )