Anomalous halogen bonds in the crystal structures of 1,2,3-tri-bromo-5-nitro-benzene and 1,3-di-bromo-2-iodo-5-nitro-benzene.

The title trihalogenated nitro-benzene derivatives, C6H2Br3NO2 and C6H2Br2INO2, crystallize in triclinic and monoclinic cells, respectively, with two mol-ecules per asymmetric unit in each case. The asymmetric unit of the tri-bromo compound features a polarized Br(δ+)⋯Br(δ-) inter-molecular halogen bond. After substitution of the Br atom in the para position with respect to the nitro group, the network of X⋯X halogen contacts is reorganized. Two inter-molecular polarized halogen bonds are then observed, which present the uncommon polarization Br(δ+)⋯I(δ-): the more electronegative site (Br) behaves as a donor and the less electronegative site (I) as an acceptor for the charge transfer.