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 et hers, 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 bond s of average distance 2.75 angstrom. The AsF6- counter-ion is perfectl y 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-degr ees-C, is tetragonal. The crystal structure shows a sandwich complex w here the oxonium is located between the crown ethers. The counter-anio n, 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 ar e disordered. Complex III, 2(12-C-4)/H3O+/AsF, m.p.150-degrees-C, is m onoclinic. This structure is also a sandwich complex, where the oxoniu m is located between two crown ethers. Only five of the eight crown et her oxygens are involved in hydrogen bonds to the oxonium ion. The dis tances O(C)-O(W) range from 2.53 to 2.84 The other three oxygens are a t distances exceeding 3 angstrom from the oxonium ion.