Synthesis and Structural Characterization of New Xenon(II) Compounds and the use of an Oxidant for the Preparation of Halogenated Carbocations

Abstract

<p>The chemistry of Xe(II) has been significantly extended to include the first examples of a neutral Xe(II) oxide fluoride species, O(XeF)₂, as well as the first nitrate derivative of Xe(II), FXeONO₂. Until recently, neutral oxide fluorides were known for all formal oxidation states of xenon except Xe(II). The synthesis of the missing oxide fluoride of Xe(II), O(XeF)₂, has been accomplished by reaction of the [FXeOXeFXeF][AsF₆] salt with NOF and characterized by NMR spectroscopy in CH₃CN solution at -78 °C and by Raman spectroscopy. Reaction of NO₂F with [FXeOXeFXeF][AsF₆] has provided the first structurally characterized noble-gas nitrate, FXeONO₂, which slowly decomposes (-78 °C) to XeF₂·N₂O₄. X-ray crystal structures have been determined for FXeONO₂, XeF2·N₂O₄, and XeF2·HNO₃. The preparation of the XeONO₂⁺ cation was attempted by the reaction of FXeONO₂ with AsF₅ at -78 °C, but was not directly observed. It is presumed that the cation initially forms, but rapidly decomposes to give Xe, O₂, and [NO₂][AsF₆].</p> <p>The salt, [XeOTeF₅][Sb(OTeF₅)₆], is a strong, low-temperature oxidant capable of oxidizing halomethanes in SO₂ClF solvent at -78 °C. The CCl₃⁺ and CBr3⁺ cations have been synthesized by oxidation of CCl₄ and CBr₄, respectively. The CBr₃⁺ cation reacts with BrOTeF₅, produced in the initial redox reaction, to give CBr(OTeF₅)₂⁺, C(OTeF₅)₃⁺ , and Br₂. The XeOTeF₅⁺ cation also reacts with BrOTeF₅ to give the Br(OTeF₅)₂⁺ cation. The X-ray crystal structures of [CCl₃][Sb(OTeF₅)₆], [CBr₃][Sb(OTeF₅)₆]•SO₂ClF, and [C(OTeF5)3][Sb(OTeF5)6]·3SO₂ClF have been determined and show that the carbocations are trigonal planar about the central atom.</p> <p>Reactions of chlorofluoro-and bromofluoromethanes with [XeOTeF₅][Sb(OTeF₅)₆] have also been investigated in SO₂ClF solvent by ¹³C and ¹⁹F NMR spectroscopy at -80 °C. The CFCl₂⁺ and CFCl(OTeF₅)⁺ cations are among the carbocations that have been obtained by reactions of CFCl₃ and CF₂Cl₂ with XeOTeF₅⁺. The CF₂Br⁺ cation is an intermediate in the reaction of XeOTeF₅⁺ with CF₂Br₂, undergoing rapid halogen exchange with CF₂Br₂ to form CFBr₂⁺ and CF₃Br. The CFBr₂⁺ cation undergoes further halogen exchange over several hours to form the CBr₃⁺ cation and CF₃Br. Although the highly electrophilic CF₃⁺ cation has not been isolated by the reaction of CF₃Br with XeOTeF₅⁺,¹³C and ¹⁹F NMR spectroscopy indicates the CF₃⁺ cation reacts with BrOTeF₅ to form F₃CBrOTeF₅⁺ and/or abstracts an OTeF₅ group from the Sb(OTeF₅)₆^-anion to yield CF₃OTeF₅ and, ultimately, [SbBr₄][Sb(OTeF₅)₆].</p> <p>The synthesis of C(OTeF₅)₄ has been accomplished by reaction of CBr4 with Br0TeF5 in SO₂ClF solution, and has been fully characterized by NMR spectroscopy, Raman spectroscopy, and single-crystal X-ray diffraction, and its geometric parameters have been compared with those of the isoelectronic B(OTeF₅)₄^-anion in order to assess the symmetry of the E(OTe)₄^-/0 (E = B, C) subgroup.</p>