The structures of crown-ether/oxonium/AsF₆^- 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°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 Å. The AsF₆^- counter-ion is perfectly octahedral at a distance of 7.1 Å from the oxonium oxygen, forming no direct contacts with the crown ether. 18-C-6 adopts the most common "D_3d-like" conformation. Complex II, 2(15-C-5)/H₃O⁺/AsF₆^-, m.p. 285°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 Å 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⁺/AsF₆^-, m.p. 150°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_CO_W range from 2.53 to 2.84 Å. The other three oxygens are at distances exceeding 3 Å from the oxonium ion.