摘要：

J. CHEM. soc. PERKIN TRANS. 2 1994 2353 The Nature of Halogen. Halogen Interactions and the Crystal Structure of V. R. Pedireddi,a D. Shekhar Reddy,a B. Satish Goud,a Donald C. Craig/ A. David Raeb and Gautam R. Desiraju*Sa a School of Chemistry, University of Hyderabad, Hyderabad 500 134, India School of Chemistry, University of New South Wales, Kensington, New South Wales, Australia 2033 ~~~~ 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 CI CI, Br-e-Br, Im-*I, CI-=aF, Br-F, I--*F, Br-CI, I-=-CI 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 8, and O2 around the X atoms in a fragment of the type C-X-X-C.Type I contacts are defined as those in which 6, = O2 while type II are defined as those in which 8, z 90"and S2 z 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 /4,/a, a = b = 7.1984(7) and c = 28.582(4) A, 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-I ,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. It has long been known that many intermolecular X X (X = Experimenta1 C1, Br and I) distances in molecular crystals are significantly Cambridge Structural Database (CSD) Analysis.-Data were less (0.14.4 A) than the sum of the accepted van der Waals retrieved from the 1993 update of Version 5.05 of the CSD (109 radii.1-3 These short distances are also associated with certain 81 6 entries) 24 for all ordered, halogen-containing crystal preferred angular geometries and have been investigated by structures (R I0.10) with an exact match between chemical a variety of experimental and theoretical method^.^-'^ These and crystallographic connectivity. Polymeric and charged X-0. X interactions are of special significance in crystal species were excluded. Geometrical calculations were per-engineering or the deliberate design of organic crystal formed on the retrieved data for various X = - - X interactions structures and supermolecules because they may be used to using QUEST3D-GSTAT, an automatic graphics non-bonded construct layered motifs which are associated with specific search program of the CSD.Duplicate hits (identified by the solid state reactivity patterns. l4-I7 We have analysed these same reference code) were removed from the calculations by interactions earlier using the Cambridge Structural Database eliminating all but the structure with the lowest R-value in each (CSD) and have deduced that short X...X intermolecular case. Unique intermolecular interactions were considered up to distances arise from specific attractive forces between X the sum of the van der Waals radii for symmetrical halogen Br 3.70 and I I 3.96 A).atoms in crystals." Others have held that these short interactions (CI distances could originate from anisotropic effects between For unsymmetrical halogen interactions, these limits were elliptically shaped X To

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