The nature of halogen ⋯ halogen interactions and the crystal structure of 1,3,5,7-tetraiodoadamantane
An analysis of halogen ⋯ halogen (X ⋯ X) intermolecular interactions in crystals, using the Cambridge Structural Database (CSD). is presented. A total of 794 crystal structures yielded 1051 contacts corresponding to symmetrical and unsymmetrical X ⋯ X interactions of the type Cl ⋯ Cl, Br ⋯ Br, I ⋯ I, Cl ⋯ F, Br ⋯ F, I ⋯ F, Br ⋯ Cl, I ⋯ Cl and I ⋯ Br. These 1051 contacts are divided mainly into two categories, type I and type II depending upon the values of the two C–X ⋯ X angles θ1and θ2around the X atoms in a fragment of the type C–X ⋯ X–C. Type I contacts are defined as those in which θ1=θ2 while type II are defined as those in which θ1≅ 90° and θ2≅ 180°. Our results indicate that as the polarisability of the X atom increases, type II contacts become more significant than type I contacts and the X ⋯ X interaction may be more nearly considered to arise from specific attractive forces between the X atoms. A number of these concepts are succinctly illustrated in the crystal structure of 1,3,5,7-tetraiodoadamantane, 1. This structure has been reported to a very limited accuracy previously and the present work reveals an unusual twinned structure for this compound wherein the geometry of the stabilising I ⋯ I interactions is retained across the twin boundary. Compound 1 is tetragonal, space group I41/a, a= b = 7.1984(7) and c= 28.582(4)Å, and Z = 4. The packing of the molecules in the crystal is controlled by I ⋯ I interactions. The supramolecular network of I ⋯ I connected molecules in crystalline 1 is closely related to that in adamantane-1,3,5,7-tetracarboxylic acid. Indeed, the stabilising nature of the I ⋯ I interactions is crucial for the crystallisation of 1 in this particular structure because otherwise, it should also have formed plastic crystals as do the analogous tetrachloro and tetrabromo derivatives.