Investigations of the polyiodides H3O center dot I-x (x=3, 5 or 7) as dibenzo-18-crown-6 complexes

The compounds [H3O(dibenzo-18-crown-6)][I-x] (x = 3,5,7) have been synthesi sed and investigated by X-ray diffraction as well as Raman and far-IR spect roscopy. The structure of the triiodide contains two independent, slightly asymmetric and bent I-3(-) ions with I-I distances in the range 2.92-2.94 A ngstrom. The pentaiodide can be described as composed of (I-3(-)). I-2, whe re the I-I distances in the asymmetric and slightly bent I-3 unit are 2.894 (2) and 2.942(2) Angstrom and in the I-2 unit 2.763(2) Angstrom. The L-shap ed I-5(-) ions are linked together forming a chain of I-10 rectangles throu gh van der Waals interactions in the range 3.7-3.8 Angstrom. The structure determination supports a pyramidal geometry of the H3O+ ion in the dibenzo- 18-crown-6 moiety of both structures. Because of severe crystal twinning th e crystal structure of the heptaiodide could not be determined. However, th e results from the spectroscopic investigation are consistent with a heptai odide of composition (I-3(-)). 2I(2).