Lewis acid properties of the XeF⁺ cation and its adducts with organic nitrogen bases

Abstract

<p>This thesis describes the synthesis and spectroscopic characterizaton of noble-gas compounds containing Xe-N bonds in solution by multinuclear magnetic resonance (multi-NMR) spectroscopy and in the solid state by low-temperature Raman spectroscopy.</p> <p>The present work represents an extension of noble-gas chemistry, and in particular, the synthesis of novel xenon-nitrogen bonded compounds. The key synthetic approach invovles the interaction of the Lewis acid XeF⁺ with an organic nitrogen Lewis base, where the organic nitrogen base must be resistant to oxidation by the XeF⁺ cation. Hydrogen Cyanide, alkyl nitriles, perfluorophenyl nitrile and perffluoropyridine derivatives were investigated as potential ligands for xenon(II). The electron lone pairs of nitriles and perfluoropyridines have been shown to interact with the Lewis acid XeF⁺ resulting in the cations RC≡N-XeF⁺ (R=H, CH₃, CH₂F, CH₂Cl, C₂H₅, CH₂FCH₂, <em>n</em>-C₃H₇, CH₂FCH₂CH₂, CH₃CHFCH₂, CHF₂CH₂CH₂, CH₃CF₂CH₂, <em>n</em>-C₄H₉, CH₃CHFCH₂CH₂, (CH₃)₂CH, (CH₃)₃C, FCH₂C(CH₃)H, ClCH₂C(CH₃)H and C₆F₅) and R_fC₅F₄N^-XeF⁺ (R_f=F, 2-CF₃, 3-CF₃, 4-CF₃). These cations have been charaterized in HF and/or BrF₅ by ¹²⁹Xe, ¹⁹F, ¹⁵N, ¹⁴N, ¹³C and ¹H NMR spectroscopy and in the solid state by low temperature Raman spectroscopy.</p> <p>The hydrogen cyanida, alkyl nitriles and perfluorophenyl nitrile adducts represent the first examples of xenon bonded to nitrogen that is formally sp hybridized. The perfluoropyridine adducts are the first examples of xenon bonded to aromatic rings.</p> <p>The thermal stabilities of RC≡N-XeF⁺AsF₆^- salts have been examined by warming HF solutions of RC≡N (R=H, CH₃, C₂H₅, <em>n</em>-C₃H₇ and <em>n</em>-C₄H₉) and XeF⁺AsF₆^- for several hours at room temperature. The stabilities of RC≡N-XeF⁺AsF₆^-salts with respect to alkyl chain fluorination depends on the chain-length of the alkyl group , decreasing in the order: <em>n</em>-C₄H₉C≡N><em>n</em>-C₃H₇C≡N>HC≡N>C₂H₅C≡N>CH₃C≡N.</p> <p>The decomposition products of HC≡N-XeF+ have been characterized by ¹H, ¹³C, ¹⁵N, ¹⁴N, and ¹⁹F NMR spectroscopy, for natural abundance 99.3% ¹³C-enriched [¹³C]HC≡N-XeF⁺AsF₆^- and 99.7% ¹⁵N-enriched [¹⁵N]HC≡N-XeF⁺AsF₆^-. Among the decomposition products that were identified were CF₃H, CF₄, CF₃NH₃⁺, CF₂=NH₂⁺ and HFC=NH₂⁺. The NMR spectra of HC≡N recorded at -15°C in HF after warming for 7 days at room temperature indicate that one species, CHF₂NH₃⁺, was formed, which presumable arises from the stepwis addition of HF.</p> <p>The solvolytic behaviours of the adducts salts, RC≡N-XeF⁺AsF₆^-, have also been studied in anhydrous HF solvent. The decomposition of the nitrile adducts cations CH₃(CH₂)_nC≡N^-XeF⁺ (n=0-3) have been monitored in HF solution by multi-NMR spectroscopy. The rate of fluorination of the alkyl chain was shown to increase with increasing chain length, where the degree of fluorination increases at the alkyl carbons in the order β<γ<δ, with no fluorination being observed at the α-Carbon. A parallel study of the alkyl nitriles RC≡N (R=CH₃, C₂H₅, <em>n</em>-C₃H₇ and <em>n</em>-C₄H₉) in HF showed the fluorination produces are significantly different. The reaction mechanisms of the flourination reactions have been proposed, i.e., the former is a radical-substitution reactions and the latter is an addition reaction and dimerization.</p>