The Syntheses of Carbocations by Use of the Noble-Gas Oxidant, [XeOTeF₅][Sb(OTeF₅)₆]:  The Syntheses and Characterization of the CX₃⁺ (X = Cl, Br, OTeF₅) and CBr(OTeF₅)₂⁺ Cations and Theoretical Studies of CX₃⁺ and BX₃ (X = F, Cl, Br, I, OTeF₅)^†

The CCl₃⁺ and CBr₃⁺ cations have been synthesized by oxidation of a halide ligand of CCl₄ and CBr₄ at −78 °C in SO₂ClF solvent by use of [XeOTeF₅][Sb(OTeF₅)₆]. The CBr₃⁺ cation reacts further with BrOTeF₅ to give CBr(OTeF₅)₂⁺, C(OTeF₅)₃⁺, and Br₂. The [XeOTeF₅][Sb(OTeF₅)₆] salt was also found to react with BrOTeF₅ in SO₂ClF solvent at −78 °C to give the Br(OTeF₅)₂⁺ cation. The CCl₃⁺, CBr₃⁺, CBr(OTeF₅)₂⁺, C(OTeF₅)₃⁺, and Br(OTeF₅)₂⁺ cations and C(OTeF₅)₄ have been characterized in SO₂ClF solution by ¹³C and/or ¹⁹F NMR spectroscopy at −78 °C. The X-ray crystal structures of the CCl₃⁺, CBr₃⁺, and C(OTeF₅)₃⁺ cations have been determined in [CCl₃][Sb(OTeF₅)₆], [CBr₃][Sb(OTeF₅)₆]·SO₂ClF, and [C(OTeF₅)₃][Sb(OTeF₅)₆]·3SO₂ClF at −173 °C. The CCl₃⁺ and CBr₃⁺ salts were stable at room temperature, whereas the CBr_n(OTeF₅)_3-n⁺ salts were stable at 0 °C for several hours. The cations were found to be trigonal planar about carbon, with the CCl₃⁺ and CBr₃⁺ cations showing no significant interactions between their carbon atoms and the fluorine atoms of the Sb(OTeF₅)₆^- anions. In constrast, the C(OTeF₅)₃⁺ cation interacts with an oxygen of each of two SO₂ClF molecules by coordination along the three-fold axis of the cation. The solid-state Raman spectra of the Sb(OTeF₅)₆^- salts of CCl₃⁺ and CBr₃⁺ have been obtained and assigned with the aid of electronic structure calculations. The CCl₃⁺ cation displays a well-resolved ³⁵Cl/³⁷Cl isotopic pattern for the symmetric CCl₃ stretch. The energy-minimized geometries, natural charges, and natural bond orders of the CCl₃⁺, CBr₃⁺, CI₃⁺, and C(OTeF₅)₃⁺ cations and of the presently unknown CF₃⁺ cation have been calculated using HF and MP2 methods have been compared with those of the isoelectronic BX₃ molecules (X = F, Cl, Br, I, and OTeF₅). The ¹³C and ¹¹B chemical shifts for CX₃⁺ (X = Cl, Br, I) and BX₃ (X = F, Cl, Br, I) were calculated by the GIAO method, and their trends were assessed in terms of paramagnetic contributions and spin−orbit coupling.