Crystal structure of di-μ-chlorido-bis(chlorido{N 1-phenyl-N 4-[(pyridin-2-yl-κN)methylidene]benzene-1,4-diamine-κN 4}mercury(II))

The whole molecule of the title complex, [Hg2Cl4(C18H15N3)2], is generated by inversion symmetry. It was synthesized from the pyridine-derived Schiff base N-phenyl-N′-[(pyridin-2-yl)methylidene]benzene-1,4-diamine (PPMBD). The five-coordinated Hg2+ ions have a distorted square-pyramidal environment defined by two N atoms, viz. the imine and the other pyridyl [Hg—N = 2.467 (6) and 2.310 (6) Å, respectively] belonging to the bidentate iminopyridine ligand, and three Cl atoms [Hg—Cl = 2.407 (2), 2.447 (2) and 3.031 (2) Å]. The longest Hg—Cl bond is bridging about the inversion centre. In the ligand, the central ring and pyridine ring are oriented at a dihedral angle of 8.1 (4)°, while the planes of the pyridine ring and the terminal phenyl ring are oriented at a dihedral angle of 53.8 (4)°. In the crystal, molecules are linked by N—H⋯Cl and C—H⋯Cl hydrogen bonds, forming sheets parallel to (001).


S1. Comment
Mercury is one of the most prevalent toxic metals in the environment and gains access to the body orally or dermally, causing cell dysfunction that consequently leads to health problems (Mandal et al., 2012). Schiff base complexes of 2pyridinecarboxaldehyde and its derivatives have been found to be good herbicides and used for the protection of plants (Hughes & Prince, 1978). Transition metal complexes of pyridyl Schiff bases have found applications in catalysis (Kasselouri et al., 1993), Pyridyl derivatives of Schiff bases are important building blocks of many important compounds widely used in biological applications such as antioxidative,anticancer, fluorescent probe agents in industry, in coordination chemistry and in catalysis (Motswainyana et al., 2013;Das et al. , 2013;Song et al. 2011;Jursic et al., 2002). The synthesis of a complex of mercury(II) using the 2-pyridincarbaldehyde derivative of the Schiff base N-phenyl-N′-pyridin-2-ylmethylene benzene-1,4-diamine (PPMBD) has not previously been reported. We report herein the crystal structure of a new mercury(II) complex of this ligand.
The whole molecule of the title complex, Fig. 1, is generated by inversion symmetry. The Schiff base derived PPMBD ligand coordinates to the Hg II atom as a bidentate ligand through the N atoms of the imine group and pyridine ring. Also two bridging and one terminal chloride anions are present in the coordination environment of the Hg II atom (Baul et al., 2004). The five-coordinated Hg 2+ ions have a distorted square-pyramidal geometry defined by two N atoms viz. one imine, the other pyridyl [Hg-N = 2.467 (6) and 2.310 (6) Å, respectively], belonging to the bidentate iminopyridine ligand and three Cl atoms [Hg-Cl = 2.407 (2), 2.447 (2) and 3.031 (2) Å]. The longest Hg-Cl distance, Hg1···Cl1 i = 3.031 (2) Å, is bridging about the centre of inversion (symmetry code: (i) -x+1, -y+1, -z+1). The observed Hg-Cl and Hg-N bond lengths and bond angles are considered normal for this type of Hg II complex (Faizi & Sen, 2014). The central ring and pyridine ring are oriented at a dihedral angle of 8.10 (6)°. The pyridine ring and terminal phenyl ring are oriented at a dihedral angle of 53.78 (6)°.
In the crystal, molecules are linked by N-H···Cl and C-H···Cl hydrogen bonds forming sheets parallel to (001); see  Table 1.

S2. Synthesis and crystallization
The iminopyridyl compound N-phenyl-N′-pyridin-2-ylmethylene benzene-1,4-diamine (PPMBD) was prepared by adding drop wise pyridine-2-carbaldehyde (0.29 g, 2.71 mmol) to a methanolic solution (50 ml) of N-phenyl-p-phenylenediamine (0.50 g, 2.71 mmol). The reaction mixture was stirred for 3 h at room temperature and filtered. The resulting yellow solid powder was washed with methanol (2 × 3 ml) and hexane (3 × 10 ml), respectively. The compound was recrystallized from in hot MeOH to give yellow crystals, which were dried in a vacuum desiccator to give the pure product The title compound was prepared by reacting (PPMBD) (0.100 g, 0.37 mmol) with mercury(II) chloride (0.099 g, 0.37 mmol) in methanol (5 ml), with vigorous stirring for 2 h at room temperature The yellow precipitate that formed was filtered off and redissolved in dimethylformamide. Crystals of the title complex suitable for X-ray analysis was obtained within 3 days by slow evaporation of the dimethylformamide. The yellow crystals of the title compound were isolated (yield: 0.31 g, 77.1%; m.p.: 520 K).

S3. Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The NH H-atom was located in difference Fourier map and refined with a distance restraint: N-H = 0.88 (2) Å with U iso (H) = 1.2U eq (N). The C-bound H-atoms were positioned geometrically and refined using a riding model: C-H = 0.95 Å with U iso (H) = 1.2U eq (C).

Figure 1
The molecular structure of the title compound, with atom labelling. Displacement ellipsoids are drawn at the 50% probability level. The unlabelled atoms are related to the labelled atoms by inversion symmetry (symmetry code: -x+1, -y+1, -z+1).

Figure 2
The crystal packing of the title compound viewed along the c axis. The hydrogen bonds are shown as dashed lines (see Table 1 for details), and for clarity only the H atoms involved in hydrogen bonding are shown.