Crystal structure of {2,6-bis[(dimethylamino)methyl]phenyl-κ3 N,C 1,N′}(bromido/chlorido)mercury(II)

The structure of C12H19Br0.30Cl0.70HgN2·C12H19Br0.24Cl0.76HgN2 contains two molecules in the asymmetric unit with each containing a mixed Cl/Br halide site. The two molecules are linked into dimers by a combination of Hg⋯Hg [Hg⋯Hg = 3.6153 (3) Å] and C—H⋯Cl and C—H⋯π interactions.


Chemical context
Organomercury compounds of type R 2 Hg and RHgX (R = alkyl or aryl; X = halide) have received considerable attention in the last three decades, mainly related to the search for versatile reagents in controlled transmetallation reactions (Wardell, 1985). Organomercury(II) derivatives have been used successfully to obtain the desired organometallic compounds of transition metals, as well as main group metals otherwise inaccessible by classical Grignard and/or lithiation reactions (Bonnardel et al., 1996;Gul & Nelson, 1999a,b;Berger et al., 2001Berger et al., , 2003Zhang et al., 2005;Djukic et al., 2006). Although the toxicity of mercury compounds should always be taken into account, there are important advantages, e.g. the possibility of preparing functionalized organomercury derivatives and the high selectivity of the transmetallation reaction (Ding et al., 1993;Pfeffer et al., 1996;Wu et al., 1998;Dreher & Leighton, 2001;Crimmins & Brown, 2004). Some cyclometallated organomercury(II) chlorides containing Ndonor functionalized aryl ligands were investigated in the context of their use as transmetallation reagents (Ali et al., 1989;Constable et al., 1989, 1991, Srivastava et al., 2010. Thus, organomercury(II) compounds serve as the precursor for the synthesis of various organometallic derivatives of transition metals, as well as main group metals, thus there is much interest in the structural characterization of these derivatives.

Structural commentary
The molecular structure of 2 is shown in Figs. 1 and 2. The compound crystallized with two molecules in the asymmetric unit. In each molecule, the halide site is mixed Cl/Br with occupancies of 0.699 (7):0.301 (7) in molecule A and 0.763 (7):0.237 (7) in molecule B. In these moieties, there are two coordination spheres around each Hg atom (Table 1). If we consider the first coordination sphere, the spatial arrangement of each Hg atom is distorted square planar with a coordination sphere made up of C-Hg-Cl/Br. Interestingly, both amine side arms are displaced from this plane in the same direction and thus both are on the same side of the phenyl ring. This displacement of the bulky groups with respect to the phenyl ring attached to Hg has been observed previously (Lau & Kochi, 1986).

Supramolecular features
A significant feature of this compound is the presence of a weak interaction between both chemically similar d 10 -d 10 metals. The distance is 3.6153 (3) Å , which is significantly smaller than the sum of the van der Waals radii for HgÁ Á ÁHg (AEr vdW =3.96 Å ; Bondi, 1964). This links the molecules into dimers which are further stabilized by both C-HÁ Á ÁCl/Br (

Figure 1
The dimeric unit formed by a combination of HgÁ Á ÁHg, C-HÁ Á ÁCl and C-HÁ Á Á interactions (all shown with dashed bonds). Only the major chloride moiety is shown. Atomic displacement parameters are at the 30% probability level.

Synthesis and crystallization
The precursor N,C,N-pincer ligand [2,6-(CH 2 NMe 2 ) 2 C 6 H 3 Br], 1, was prepared according to the procedure given by van Koten and co-workers (van de Kuil et al., 1994) with slight modifications. An excess of HNMe 2 (in H 2 O) was employed instead of 2.2 equivalents to quench with 2-bromo-1,3-bis-(bromomethyl)benzene. This afforded a yellow oil which was purified by vacuum distillation to give a colorless oil in 70% yield. n-BuLi (1.15 ml, 1.84 mmol) was added dropwise via syringe to the solution of 1 (0.50 g, 1.84 mmol) in dry Et 2 O (15 ml) under an inert atmosphere at 273 K. After 30 min, the color of the reaction mixture changed from colorless to pale yellow. To this, a solution of HgCl 2 (0.50 g, 1.84 mmol) in dry THF (10 ml) was added. The whole mixture was stirred for 5 h at 273 K and then allowed to warm slowly to room temperature. Then reaction mixture was filtered and the filtrate evaporated to dryness and the resulting precipitate extracted with hexane. The workup afforded a white precipitate of 2 (yield 0.36 g, 75%; m.p. 408-410 K). Colorless crystals of 2 suitable for single-crystal diffraction analysis were obtained by slow diffusion of hexane into CHCl 3 at room temperature.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms were positioned geometrically and allowed to ride on their parent atoms, with C-H = 0.95-0.99 Å and U iso (H) = xU eq (C), where x = 1.5 for methyl H atoms and 1.2 for all other C-bound H atoms. There are two molecules in the asymmetric unit and in each the halide site is occupied by a mix or Cl and Br, with refined occupancies of 0.699 (7):0.301 (7) and 0.763 (7):0.237 (7), respectively.

Funding information
RJB is grateful for the NSF award 1205608, Partnership for Reduced Dimensional Materials, for partial funding of this research, as well as the Howard University Nanoscience Facility access to liquid nitrogen. RJB acknowledges the NSF MRI program (grant No. CHE-0619278) for funds to purchase an X-ray diffractometer. HBS is grateful to Department of Science and Technology, New Delhi, for a J. C. Bose Fellowship. Computer programs: CrysAlis PRO (Agilent, (2012), SHELXS97 and SHELXTL (Sheldrick, 2008) and SHELXL2017 (Sheldrick, 2015). Table 2 Hydrogen-bond geometry (Å , ).  (Agilent,(2012); cell refinement: CrysAlis PRO (Agilent,(2012); data reduction: CrysAlis PRO (Agilent,(2012); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2017 (Sheldrick, 2015); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.002 Δρ max = 1.28 e Å −3 Δρ min = −1.66 e Å −3 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.