Crystal structure of (N 1-benzyl-N 1,N 2,N 2-trimethylethane-1,2-diamine-κ2 N,N′)dichloridomercury(II)

The molecular structure of [HgCl2(C12H20N2)] is a rare example where the HgII atom is bound to a Cl2N2 donor set for which the N atoms originate from aliphatic tertiary amine groups.


Chemical context
The chemistry of mercuric compounds with multidentate amine ligands is of interest due to the low coordination number and geometry preferences of Hg II , which facilitates extraordinarily rapid exchange of simple ligands Carra et al., 2013). The enhanced binding thermodynamics of these multidentate ligands has been used to suppress intermolecular ligand-exchange rates for a variety of Hg II complexes in solution, greatly enhancing the meaningfulness of NMR characterization. Significantly, under conditions of slow intermolecular exchange the rates of intramolecular isomerization processes for Hg II can still exceed both the chemical shift and coupling constant time scale, particularly when bond cleavage is unnecessary and structures of these complexes have been determined Carra et al., 2013).

Structural commentary
In the structure of (3), the Hg II atom is four-coordinated by two tertiary amine N-atom donors, as well as two Cl À anions to give a distorted tetrahedral coordination environment (Fig. 2). The distortion from ideal values can be seen by the dihedral angle between the N1-Hg-N2 and Cl1-Hg-Cl2 planes of 82.80 (9) . The Hg-N and Hg-Cl bond lengths are in the normal ranges for such bonds (Allen, 2002). The fivemembered chelate ring adopts an envelope conformation with puckering parameters of Q(2) = 0.446 (6)Å and '(2) = 88.8 (6) (Cremer & Pople, 1975), with the two amine CH 3 substituents on opposite sides of the ring. Of the two reported structures which contain Hg II attached to tertiary N donors (Choi et al., 2005;Niu et al., 2004), only one has Hg II in an N 2 Cl 2 coordination environment (Choi et al., 2005) and thus provides the best comparison. The Hg-Cl [2.3875 (14) and 2.4397 (13) Å ] and Hg-N bond lengths [2.355 (4) and 2.411 (4) Å ] in (3) agree well with those found in the previous example [Hg-Cl = 2.397 (3) and 2.374 (2) Å ; Hg-N = 2.353 (7) and 2.391 (6) Å ].

Supramolecular features
The molecular adducts are linked by C-HÁ Á ÁCl interactions (Table 1 and Fig. 3) into a zigzag chain parallel to [101]. As a result of the bulky nature of the complex, with the two amine CH 3 substituents on opposite sides of the chelate ring, there is no evidence of anyinteractions.

Database survey
In view of the interest in the coordination chemistry of mercury, it is surprising that a search of the Cambridge Structural Database (Version 5.35, November 2013 with one update; Allen, 2002) for structures of Hg II with an N 2 Cl 2 coordination sphere gave 96 hits, but the vast majority of these involved aromatic N donors such as pyridine and imidazole. There were only six hits involving aliphatic amine N-atom donors and only two (Choi et al., 2005;Niu et al., 2004) where the N atoms involved were both from tertiary amine functionalities. The molecular packing for [HgCl 2 (C 12 H 20 N 2 )] viewed along the c axis. C-HÁ Á ÁCl interactions are shown as dashed lines.

Figure 1
Reaction scheme showing the synthesis of the title compound.

Synthesis and crystallization
A stirred solution of N 1 -(2-bromobenzyl)-N 1 ,N 2 ,N 2 -trimethylethane-1,2-diamine, (1), (1.10 ml, 5.34 mmol) in dry THF (15 ml) was treated dropwise with a 1.6 M solution of n-BuLi in hexane (3.80 ml, 6.15 mmol) via syringe under N 2 at 273 K. On stirring the reaction mixture for 2 h at this temperature, the lithiated product (2) was obtained. Mercuric chloride (1.55 g, 5.70 mmol) was added to the reaction mixture under a brisk flow of N 2 gas and stirring was continued for an additional 6 h at room temperature. The reaction mixture was then removed from the N 2 line and evaporated to dryness to give a colourless hygroscopic solid. The solid was extracted with dry chloroform. The organic phase was separated, dried over Na 2 SO 4 , and filtered. The filtrate was evaporated to dryness to give a colourless crystalline solid of the HgCl 2 adduct of N 1 -benzyl-N 1 ,N 2 ,N 2 -trimethylethane-1,2-diamine, (3) (yield 1.25 g, 51%). The reaction scheme is shown in Fig. 1.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms, with C-H distances of 0.95 (aromatic) and 0.99 Å (methylene), with U iso (H) = 1.2U eq (C), and C-H = 0.98 Å for methyl H atoms, with U iso (H) = 1.5U eq (C).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 > σ(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.