Crystal structures of complexes between alkali-metal salts and cyclic polyethers. Part V. The 1 : 2 complex formed between potassium iodide and 2,3,5,6,8,9,11,12-octahydro-1,4,7,10,13-benzopentaoxacyclopentadecin (benzo-15-crown-5)

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Abstract
Potassium iodide forms a 1 : 2 complex with the cyclic polyether ‘benzo-15-crown-5’. The crystals are tetragonal with a=b= 17·84(1), c= 9·750(6)Å, Z= 4, space group P4/n. The crystal structure has been determined by the heavy-atom method and refined by full-matrix least-squares to a final R of 0·09 on 1603 diffractometer data. The complex cation has crystallographic symmetry , the potassium being ‘sandwiched’ between two centro-symmetrically related ligand molecules. In each of these the five ether-oxygen atoms are approximately coplanar, the cation lying 1·67 Å away from this plane, so that the ten-co-ordination is an irregular pentagonal antiprism. The K–O distances range from 2·777(7) to 2·955(8)Å. Iodide ions occupy two sets of positions with 4 or symmetry and the arrangement of complex cations and anions resembles the caesium chloride structure; there is no interaction between the anions and the metal.