Stable Chloro- and Bromoxenate Cage Anions; [X3(XeO3)3]3– and [X4(XeO3)4]4– (X = Cl or Br)

The number of isolable compounds which contain different noble-gas–element bonds is limited for xenon and even more so for krypton. Examples of Xe–Cl bonds are rare, and prior to this work, no Xe–Br bonded compound had been isolated in macroscopic quantities. The syntheses, isolation, and characterization of the first compounds to contain Xe–Br bonds and their chlorine analogues are described in the present work. The reactions of XeO3 with [N­(CH3)4]Br and [N­(C2H5)4]­Br have provided two bromo­xenate salts, [N­(C2H5)4]3[Br3(XeO3)3] and [N(CH3)4]4[Br4(XeO3)4], in which the cage anions have Xe–Br bond lengths that range from 3.0838(3) to 3.3181(8) Å. The isostructural chloroxenate anions (Xe–Cl bond lengths, 2.9316(2) to 3.101(4) Å) were synthesized by analogy with their bromine analogues. The bromo- and chloroxenate salts are stable in the atmosphere at room temperature and were characterized in the solid state by Raman spectroscopy and low-temperature single-crystal X-ray diffraction, and in the gas phase by quantum-chemical calculations. They are the only known examples of cage anions that contain a noble-gas element. The Xe–Br and Xe–Cl bonds are very weakly covalent and can be viewed as σ-hole interactions, similar to those encountered in halogen bonding. However, the halogen atoms in these cases are valence electron lone pair donors, and the σ*Xe–O orbitals are lone pair acceptors.