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

The title dinuclear mercury(II) complex, [Hg2Cl4(C16H19N3)2], synthesized from the pyridine-derived Schiff base (E)-N 1,N 1-diethyl-N 4-[(pyridin-2-yl)methylidene]benzene-1,4-diamine (DPMBD), has inversion symmetry with the five-coordinated HgII centres having distorted square-pyramidal stereochemistry comprising two N-atom donors from a bidentate chelate BPMBD ligand and three Cl-atom donors, one monodentate and two bridging.


Chemical context
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, 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 for many important compounds, widely used in biological applications such as antioxidative, anticancer agents, as fluorescent probes in industry, in coordination chemistry and in catalysis (Jursic et al., 2002;Song et al., 2011;Motswainyana et al., 2013;Das et al., 2013). Our research interest focuses on a study of Schiff bases derived from N 1 ,N 1 -diethyl-p-phenylenediamine and their metal complexes (Faizi & Hussain, 2014;. We report herein the synthesis and the crystal structure of a new complex of mercury(II), [Hg 2 Cl 4 (C 16 H 19 N 3 ) 2 ], with the pyridinederived Schiff base (E)-N 1 ,N 1 -diethyl-N 4 -[(pyridin-2-yl)methylidene]benzene-1,4-diamine (DPMBD).

Structural commentary
The dinuclear molecule of the title complex is generated by inversion symmetry (Fig. 1). The Schiff base-derived ligand (DPMBD) coordinates to the Hg II atom in a bidentate ISSN 2056-9890 chelating mode through the N atoms of the pyridine ring (N1) and the imine group (N2) [Hg1-N = 2.317 (9) and 2.437 (8) Å , respectively].

Figure 2
The crystal packing of the title compound, viewed along the c axis, with hydrogen bonds (Table 1) shown as dashed lines. (Faizi & Prisyazhnaya, 2015) and di-chlorido-bis-(chlorido{N 1 -phenyl-N 4 )-[(pyridin-2-yl-N)methylidene]benzene-1,4-diamine-N 4 } mercury(II)). All of the above compounds show the Hg II ion in a distorted squarepyramidal coordination environment formed by the N atoms of the diimine ligand, two bridging Cl atoms and one monodentate Cl atom, as found in the title compound, one of the bridging Hg-Cl bonds being significantly longer than the other.
The title compound was prepared by reacting DPMBD (0.10 g, 0.39 mmol) with mercury(II) chloride (0.05 g, 0.18 mmol) in methanol (5 ml), with vigorous stirring for 2 h at room temperature. The red precipitate that formed was filtered off and redissolved in dimethylformamide. Crystals of the red title complex (yield: 0.31 g, 76%) suitable for X-ray analysis were obtained within 3 d by slow evaporation of the dimethylformamide.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms bonded to C atoms were placed in calculated positions with C-H = 0.93-0.97 Å and included in the refinement in a riding-model approximation with U iso (H) = 1.5U eq (C) (for methyl H) and U iso (H) = 1.2U eq (C) (for other H atoms). Computer programs: APEX2 and SAINT (Bruker, 2003), SHELXS97 (Sheldrick, 2008), SHELXL2016 (Sheldrick, 2015) and DIAMOND (Brandenburg & Putz, 2006).

Figure 3
The crystal packing of the title compound, viewed approximately along the c axis. Theinteractions between the benzene and pyridine rings are shown as dotted lines.