Computational and Crystallographic Studies of Pseudo-Polyhalides

Pseudo-polyhalides represent a relatively new class of compounds in which the dihalogen molecules of the polyhalide ions are replaced by organohalogen molecules. Like polyhalides, the anion structure is strongly dependent on the size, shape, and charge of the countercation, but with additional variables provided by the specific organohalogen used. The charge transfer contribution to the halogen bonding interaction in polyhalides can be quite large but is much less so in the pseudo-polyhalides. Density functional calculations show that the interaction is primarily electrostatic. The charge-transfer contribution generally increases with increasing halogen bond strength but is not enough to cause significant angular dependence at the halide ion. The structures of 10 pseudo-polyiodide salts with either cryptand[2.2.2] (K222) or hexaethyleneglycol (HEG) encrypted alkali metal cations and tetraiodoethylene as the organohalogen are reported. Three salts obtained with M­(K222)+ have widely different structures ranging from steeply corrugated layers to more planar layers with pendant groups to a three-dimensional network. Identical stoichiometries with different cations (M­(K222)+ or M­(HEG)+) give a layered or network solid, respectively, which are directly related. The latter has the unusual property of a completely disordered cationic phase, located within a perfect honeycomb anionic network.