THE NATURE OF HALOGEN-CENTER-DOT-CENTER-DOT-CENTER-DOT-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 theta(1) and theta(2) around the X atoms in a fragment of the type C-X... X-C. Type I contacts are defined as those in which theta(1) = theta(2) while type II are defined as those in which theta(1) congruent to 90 degrees and theta(2) congruent to 180 degrees. 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 /4(1)/a, a = b = 7.1984(7) and c = 28.582(4) Angstrom, 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.