Crystal structure of bis(benzimidazolium) tetrabromocuprate(II)

Stable, dark-violet crystals of (C₇H₇N₂)₂CuBr₄ were obtained from an aqueous solution of CuBr₂, HBr, and benzimidazole. The crystal structure was determined by three-dimensional X-ray analysis. The crystals are monoclinic:C2/c, a=15.651(4),b=7.945(1),c=15.775(4) Å,=91.44(2)°,Z=4,V_c=1960.96 ų,D_x=2.105 g cm^–3. The structure was refined by full-matrix least squares to giveR=0.042 andR_w=0.048 for 1254 intensities above 3(I). The copper(II) ions are four-coordinated in the form of a distorted tetrahedron CuBr₄ ofC₂ symmetry. Two different Cu-Br bond lengths are observed: Cu-Br(1)=2.426(2), Cu-Br(2)=2.330(2) Å. Only one flattened angle [Br(2)-Cu-Br(2)=132.71(9)°] in the CuBr₄ tetrahedron is noted. The angles Br(1)-Cu-Br(1)=108.83(7), Br(2)-Cu-Br(1)=99.92(5) and Br(2)-Cu-Br(1)=107.13(5)° have been found. Hydrogen bonds explain the deformation of the CuBr₄^2– coordination polyhedron. The structure is compared with other CuX₄^2– complexes.