Structure of the mercury(II) mixed-halide (Br/Cl) complex of 2,2′-(5-tert-butyl-1,3-phenylene)bis(1-pentyl-1H-benzo[d]imidazole)

The structure of the mixed halide (Br/Cl) mercury complex of 2,2′-(5-tert-butyl-1,3-phenylene)bis(1-pentyl-1H-benzimidazole) is a one-dimensional helical polymer.


Structural commentary
The title compound crystallizes in the orthorhombic space group Pca2 1 and is a racemic twin [BASF = 0.402 (9)]. The geometry around the Hg II atom is distorted tetrahedral with the Hg II atom coordinated to two N atoms, one Br atom, and a fourth coordination site is occupied by a mixed halide (Br/Cl). There are two such sites in the asymmetric unit and in one site, the Br/Cl occupancy is 0.599 (9):0.401 (9) and in the other site the ratio is 0.431 (9):0.569 (9), giving an overall composition (L) 2 HgBr 1.52 Cl 0.48 , where L is 2,2 0 -(5-tert-butyl-1,3-phenylene)bis(1-pentyl-1H-benzimidazole). Unlike the case with the recently published structure (Rani et al., 2017) of a related ligand where both coordinating positions are blocked by substituents (Br and tert-butyl), even though in the present ligand one position is open for the formation of an Hg-C bond, the basic arrangement is the same as in the previous structure.
The discussion of the conformations adopted by the ligand will be restricted to just those of the major components for the disordered moieties where the ligand has adopted a conformation where the two sets of two benzimidazole moieties are not coplanar with each other or their central arene ring. The dihedral angles between the two sets of benzimidazole moieties are 76.4 (3) and 88.0 (2) ( Table 1), while their dihedral angles with the central ring are 60.1 (3)/59.5 (3) and 50.1 (3)/60.2 (2) . These values are significantly different from those observed in the related complex (Rani et al., 2017), where the corresponding angles are 54.9 (6) and 65.6 (4) between the two benzimidazole moieties (two values due to the disorder of one of these moieties) and 55.6 (1) and 89.3 (5)/79.2 (5) with the central ring. Further, the four pentyl side chains have adopted quite different conformations. This can be seen by examining the dihedral angles of the C atoms in each of these groups. There are four such angles for each, C(from the planar benzimidazole ring)-N-C-C, N-C-C-C, C-C-C-C, and C-C-C-C (last C is a terminal CH 3 group). The four sets of values are À115.6 (11)/ À166.7 (11)/76.2 (19)/67.5 (19), 102.2 (12)/À61.5 (15)/ À171.9 (13)/173.0 (16) , 113.4 (10)/162.5 (9)/À174.7 (10)/ À179.5 (11) and À104.2 (15)/À172.4 (13)/À78.2 (15)/ 177.1 (17) . From these values it can be seen that the first two pentyl groups have adopted a slightly curled-up conformation, while the last two are in a fully extended all-trans conformation (Fig. 3).

Supramolecular features
The molecules form a one-dimensional helical polymer propagating in the b direction (Fig. 4). The helices are held together by intra-strand C-HÁ Á ÁBr and C-HÁ Á ÁCl interactions. Each strand is further linked by inter-strand C-HÁ Á ÁBr and C-HÁ Á ÁCl interactions. There are inter-strand interactions between the benzimidazole moieties which help stabilize the packing [N1B/N2B/C1B/C2B/C7BÁ Á ÁC24A-C29A (symmetry code: 1 2 + x, 1 À y, z) and N3A/N4A/C23A/ C24A/C29AÁ Á ÁC2B-C7B (symmetry code: x À 1 2 , 1 À y, z]. In addition, there are weak C-HÁ Á ÁN interactions which further stabilize the structural arrangement. Numerical details are given in Table 2 Diagram containing two units of the polymer illustrating its zigzag nature, with H atoms omitted for clarity (symmetry code for generating the second unit: x, y À 1, z). Minor components of both the pentyl and benzimidazole disorder, as well as the chloride-anion disorder, have been omitted for clarity. H atoms have also been omitted for clarity. Atomic displacement parameters are shown at the 30% probability level.

Synthesis and crystallization
To a solution of 1 (0.5 g, 0.67 mmol) in dry acetone was added tetra-n-butylammonium bromide (0.26 g, 0.82 mmol) at room temperature. The reaction mixture was refluxed for 3 h under an inert atmosphere of N 2 and then filtered through Whatman filter paper. The residue contained shiny mercury particles which could be witnessed with the naked eye and the solvent was evaporated from the filtrate and washed with petroleum ether and then with diethyl ether.

Refinement
Crystal data, data collection and structure refinement details are summarized in  Table 2 Hydrogen-bond geometry (Å , ).

Figure 4
Packing diagram, viewed along the c axis, showing two strands of the polymer which propagate in the b-axis direction.

benzimidazole)-κ 2 N 3 :N 3′ ]
Crystal data  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.