Synthesis and structure of the mercury chloride complex of 2,2′-(2-bromo-5-tert-butyl-1,3-phenylene)bis(1-methyl-1H-benzimidazole)

In the title mercury complex, the HgII atom is coordinated by two Cl atoms and by two N atoms from two 2,2′-(2-bromo-5-tert-butyl-1,3-phenylene)bis(1-methyl-1H-benzimidazole) ligands, which gives rise to a zigzag helical 1-D polymer propagating along the b-axis direction.


Structural commentary
The structure of 2 with empirical formula, C 26 H 25 BrCl 2 HgN 4 , is reported in this paper. As a result of the presence of the Br and t-butyl substituents on the central ring, coordination of the Hg II atom to this ring is prevented and thus a monomeric complex is formed, as has previously been observed for an HgCl 2 complex with a similar ligand but with a central pyridine ring rather than a phenyl ring .
Another related structure has recently been reported of a dinuclear structure of HgCl 2 with a similar ligand to 1 where there is a methyl substituent on the C1 atom of the imidazole ring . In the case of 2, however, a zigzag polymeric structure forms in the b-axis direction, in which the HgCl 2 moiety is linked by atoms N1 from one ligand and N3 from an adjoining ligand. The coordination environment around the mercury atom is distorted tetrahedral with bond angles ranging from 100.6 (2) to 126.35 (7) (Fig. 2). The two Hg-N bond lengths are equivalent at 2.333 (4) and 2.338 (4) Å . However, the metal-halogen bonds are not similar [Hg-Cl1 = 2.4424 (13) and Hg-Cl2 = 2.4020 (15) Å ]. The ligand adopts a conformation whereby the two benzimidazole moieties are not coplanar with each other or the central phenyl ring. The dihedral angles between the benzimidazole moieties N1/N2/C1-C7 and N3/N4/C19-C24 are 60.9 (2) while they make dihedral angles of 55.6 (2) and 84.2 (2) , respectively, with the central ring.

Supramolecular features
The combination of HgCl 2 with 2,2 0 -(2-bromo-5-tert-butyl-1,3phenylene)bis(1-methyl-1H-benzimidazole) results in a zigzag helical 1-D coordination polymer that propagates along the b-axis direction. This is mediated by the HgCl 2 moiety, which is linked by atoms N1 from one ligand and N3 from an adjoining ligand (Fig. 2). Although helices are inherently chiral in nature, the overall structure is not chiral as the individual helices are related by a center of inversion. The internal structure of this polymer is stabilized by both C-HÁ Á ÁCl and C-HÁ Á ÁN interactions (Table 1). In addition, there are both C-HÁ Á Á (Table 1) andinteractions [Cg6Á Á ÁCg6(1 À x, Ày, Àz) = 3.531 (2) Å , where Cg6 is the centroid of the benzimidazole ring system N3/N4/C19-C24 and C25]. There are no halogen bonds or C-HÁ Á ÁBr interactions present. Apart from van der Waals interactions, there are no significant interactions between the zigzag chains of the coordination polymer (Fig. 3). Diagram showing the three units which assemble to form a coordination polymer and illustrating its zigzag helical nature (with H atoms omitted for clarity). Displacement parameters are drawn at the 30% probability level. [Symmetry codes:

Figure 1
Diagram showing the the starting compound, 1, the title compound, 2, and the expected product, 3.
is no report in the literature for a mercury complex with 2,2 0 -(2-bromo-5-tert-butyl-1,3-phenylene)bis(1-methyl-1Hbenzimidazole) that has been structurally characterized. A cadmium complex, bis[1,3-bis(benzimidazol-2-yl)benzene]dichloridocadmium(II), in which the Cd is coordinated by two Cl atoms and two N atoms in a distorted tetrahedral configuration has been reported (Jiang et al., 2010). In the title complex 2, cadmium is replaced by an Hg II atom along with a slight modification of the ligand.

Synthesis and crystallization
To a solution of 1 (0.2 g, 0.42 mmol) in THF (15 ml) was added dropwise a solution of n-BuLi (0.3 ml, 0.47 mmol) at 195 K. The synthesis of compound 1 will be published elsewhere. The reaction mixture turned blue after immediate addition of n-BuLi. The reaction mixture was stirred for 30 min at 195 K followed by the addition of HgCl 2 (0.126 g, 0.466 mmol). The reaction mixture was warmed to room temperature and stirred for 16 h.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms were positioned geometrically and refined as riding: C-H = 0.95-0.98 Å with U iso (H) = 1.2U eq (C) or 1.5U eq (C) for methyl H atoms. Packing diagram showing two units of the polymer, which repeat in the baxis direction, viewed along the a axis. Table 1 Hydrogen-bond geometry (Å , ).

benzimidazole)-κ 2 N 3 :N 3′ ]
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 1.34 e Å −3 Δρ min = −1.88 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Hg 0.26669 (2)