Dibromido[1,1′-dibutyl-2,2′-(pentane-1,1-diyl)di-1H-benzimidazole]copper(II)

In the title compound, [CuBr2(C27H36N4)], the CuII ion exhibits a distorted tetrahedral coordination geometry provided by two bromide ions and by chelation of two imine N-atom donors from a bis(benzimidazole) ligand. Chelation results in a six-membered boat-shaped ring which links the benzimidazole groups. Each bis(benzimidazole) fragment contains three n-butyl substituents, two of which have the expected trans conformation; the third exhibits the higher-energy cis conformation, an orientation consistent with several short intramolecular C—H⋯Br interactions. Essentially planar (r.m.s. deviations of 0.0101 and 0.0183 Å) benzimidazole groups are oriented so as to give the bis(benzimidazole) fragment a V-shaped appearance in profile with the cis and trans n-butyl groups directed to opposite sides of the planes. In the crystal, columns of molecules along the b-axis direction form layers parallel to the (202) planes. Within a given column, the molecules are linked by C—H⋯Br hydrogen bonds. The molecules in adjacent columns are also linked by intermolecular C—H⋯π interactions, forming a three-dimensional network.

In the title compound, [CuBr 2 (C 27 H 36 N 4 )], the Cu II ion exhibits a distorted tetrahedral coordination geometry provided by two bromide ions and by chelation of two imine N-atom donors from a bis(benzimidazole) ligand. Chelation results in a six-membered boat-shaped ring which links the benzimidazole groups. Each bis(benzimidazole) fragment contains three n-butyl substituents, two of which have the expected trans conformation; the third exhibits the higherenergy cis conformation, an orientation consistent with several short intramolecular C-HÁ Á ÁBr interactions. Essentially planar (r.m.s. deviations of 0.0101 and 0.0183 Å ) benzimidazole groups are oriented so as to give the bis(benzimidazole) fragment a V-shaped appearance in profile with the cis and trans n-butyl groups directed to opposite sides of the planes. In the crystal, columns of molecules along the b-axis direction form layers parallel to the (202) planes. Within a given column, the molecules are linked by C-HÁ Á ÁBr hydrogen bonds. The molecules in adjacent columns are also linked by intermolecular C-HÁ Á Á interactions, forming a threedimensional network.

Comment
The title compound (I) was prepared as part of our long-term interest in the chemistry of bis(imidazoles), bis(benzimidazoles), and their complexes with metal ions. These species have demonstrated their usefulness as proton sponges (Stibrany et al., 2002), geometrically constraining ligands (Stibrany et al., 2004), agents to study electron transfer (Knapp et al., 1990), polymerization catalysts (Stibrany et al., 2003), and in the formation of metal-organic copolymers (Stibrany & Potenza, 2008).
The structure of [1,1'-bis(1-butylbenzimidazol-2-yl) pentane]copper(II) dibromide, (I), contains molecules ( Fig. 1) in which two essentially planar benzimidazole fragments are linked by the a bridging (bridgehead) carbon atom C1 and a Cu(II) ion, which forms Cu-N(imine) bonds to N13 and N23, to complete a six-membered Cu1-N13-C12-C1-C22-N23ring. The ring adopts a boat conformation with the copper(II) ion and the bridgehead carbon atom corresponding to the bow and stern, respectively. The angles N23-Cu1-N13 and C22-C1-C12 (Table 1), at the bow and stern, respectively, give the molecule a V-shape in profile (Fig. 2). Two bromine atoms, Br1 and Br2, complete a distorted-teterrahedral coordination geometry at Cu1, as evidenced by the several angles at Cu1 (Table 1) and by the "tetrahedral twist dihedral angle" N13-Cu1-N23/Br1-Cu1-Br2, 65.08 (6)°. Of the three n-butyl groups, two exhibit the trans conformation and extend above the planes of the benzimidazole fragments ( Fig. 1), while the third, bonded to the bridgehead carbon atom C1, exhibits the higher-energy cis conformation and is positioned below the planes of the benzimidazole rings. The cis orientation is consistent with several intramolecular C-H···Br interactions whose H···Br (Br2···H2B, 3.1147 Å and Br2···H4A, 3.6145 Å) distances are too long to be considered hydrogen bonds, yet too short to be ignored. Lastly, we note that the complex exhibits an intramolecular C14-H14···Br1 hydrogen bond (Table 2).
In the crystal, molecules of (I) form columns along the b cell direction ( Fig. 2) centered about the twofold screw axes at 1/4 b 1/4 and symmetry related positions in space group P2 1 /n. Within a given column, the molecules are linked by C18-H18b···Br1 hydrogen bonds ( Fig. 3) to give each column spirial staircase appearance along its length. The columns are arranged in layers parallel to the (2 0 2) planes ( Fig.2), and are linked together by intermolecular C17-H17···Br2 hydrogen bonds ( Fig. 4) to yield a three-dimensional network structure. The C-H and H···Br distances for the C-H···Br hydrogen bonds in (I) ( Table 2) compare favorably with those reported previously for a distorted-tetrahedral Cu(I) bromide complex (Hou et al., 2006).
In related structures, alkyl chains, substituted at the N(amine) and bridgehead positions of bis(benzimidazoles), have been observed in three permutations with respect to the benzimidazole planes: all to one side, two up, bridgehead substituent down as in the present instance, and two up, N(amine) substituent down (Stibrany, 2009). In the structure of the free ligand of (I) (Stibrany et al., 2003), all three alkyl chains assume the trans conformation. Presumably, the way in which these molecules pack in a crystal determines to some extent the conformation of these substituents, or vice versa. In the analogous dichloride complex, the alkyl chains are arranged similarly to those in (I) (Stibrany et al., 2003). In fact, (I) and its dichloro analogue are isomorphous.  (Stibrany et al., 2003) and 107 mg (0.48 mmol) of CuBr 2 to a mixture of 20 ml of ethanol and 2 ml of triethylorthoformate. This mixture was warmed gently for 5 min and then allowed to evaporate slowly. When the volume was reduced by approximately 60%, dark red crystals of (I) had formed and were collected by filtration, and dried in air. Yield 301 mg

Refinement
Hydrogen atoms were positioned geometrically using a riding model, with C-H = 0.95 and 1.00 Å, respectively, for n-butyl and benzimidazole H atom, and U iso (H) = 1.2-1.5 U eq (C).