THE STRUCTURES OF CROWN-ETHER OXONIUM/ASF(6)- SPECIES

The reaction of XeF+AsF6- with a series of crown ethers (containing ca 0.1 weight % H2O) resulted in three new oxonium complexes of crown ethers, two of whose structures have been determined in detail by X-ray crystallography. Complex I, 18-C-6/H3O+/AsF6-, m.p. 180-degrees-C, is orthorhombic. In this structure the oxonium occupies the centre of the crown cavity, coordinated to its six ether oxygens with hydrogen bonds of average distance 2.75 angstrom. The AsF6- counter-ion is perfectly octahedral at a distance of 7.1 from the oxonium oxygen, forming no direct contacts with the crown ether. 18-C-6 adopts the most common ''D3d-like'' conformation. Complex II, 2(15-C-5)/H3O+/AsF, m.p. 285-degrees-C, is tetragonal. The crystal structure shows a sandwich complex where the oxonium is located between the crown ethers. The counter-anion, AsF6-, is 8.0 angstrom from the oxygen atom of the oxonium and has no direct contacts with the crown ethers. The crown ether molecules are disordered. Complex III, 2(12-C-4)/H3O+/AsF, m.p.150-degrees-C, is monoclinic. This structure is also a sandwich complex, where the oxonium is located between two crown ethers. Only five of the eight crown ether oxygens are involved in hydrogen bonds to the oxonium ion. The distances O(C)-O(W) range from 2.53 to 2.84 The other three oxygens are at distances exceeding 3 angstrom from the oxonium ion.