Superacid studies : syntheses, structures and solution studies of Sb(III) and Sb(V) fluoro fluorosulfato derivatives

• Main Author: Zhang, Dingliang
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/7467

The HSO₃F/SbF₅ system (“Magic Acid”) was investigated by high resolution ¹⁹F 1D and 2D NMR (COSY and J-resolved). Twelve fluoro-fluorosulfato-antimonate(V) species were clearly identified and the relative concentrations of these species were estimated based on the integration of the ¹⁹F NMR signals for the HSO₃F/SbF₅ systems with SbF₅ concentration ranging from X[sub SbF₅]=0.000999 to X[sub SbF₅]=0.342. The 1:1 and 1:2 complexes between HSO₃F and SbF₅, [SbF₅(HSO₃F)]⁻ and [Sb₂F₁₀(μ-SO₃F)⁻, were found to be the principal species in the system. Other constituent species including the monomers, [SbF₆]⁻,[SbF₄(SO₃F)₂], [SbF₃(SO₃F)₃]⁻ as well as the dimers, [Sb₂F₁₁], [Sb₂F₉(μ-SO₃F)₂]⁻, and [Sb₂F₈(μ-SO₃F)(SO₃F)₂]⁻, were also found in the HSO₃F/SbF₅ system. Trimers of the type [SbF₅-(μ-SO₃F)-SbF₄-(μ-SO₃F)-SbF₅]⁻ are present in systems with X[sub SbF₅]≥0.0989. All the oligomeric fluoro-fluorosulfato-antimonate(V) species were found to be SO₃F-bridged rather than F-bridged. Based on the relative concentrations of the species in the HSO₃F/SbF₅ system, it appears that solvolysis of SbF₅ in HSO₃F is likely to occur in addition to the ligand redistribution. The byproduct HF reacts with SiO₂ to produce H₂0, which is protonated to give the oxonium ion H₃O⁺, which was observed in ¹H NMR spectra of the system. An oxonium salt [H₃O][Sb₂F₁₁] was crystallized from the HSO₃F/SbF₅ system with high SbF₅ content (x[sub SbF₅]≥0.342). The structure of[H₃O][Sb₂F₁₁] is determined by single crystal X-ray diffraction. It has a very complex structure and the asymmetrical O-H⋅⋅⋅F hydrogen bonds link H₃O⁺ and [Sb₂F₁₁] ions to form a 3-dimension network. Two new Sb(III) compounds, [SbF₂(SO₃F)][sub x] and [SbF(SO₃F)₂][sub x], as well as [Sb(SO₃F)₃][sub x], were prepared and their polymeric structures were determined to high precision by single crystal X-ray diffraction. A detailed comparison for the series [SbF{sub n}(SO₃F){sub 3-n}][sub x] (n = 0, 1, 2, or 3) provides an insight into the complex coordination chemistry of Sb(III) fluoride fluorosulfates. Fluoride functions as an asymmetrical bridging ligand and fluorosulfate functions as an 0-tridentate asymmetrical bridging ligands. The overall coordination number of antimony depends on the available donor sites of the ligands and accordingly increases gradually from six for SbF₃ to nine for [Sb(SO₃F)₃][sub x].The Sb-O and Sb-F interactions in all four compounds span a wide range from normal covalent bonds (1.9∼2.1 Å) to long intermolecular contact (∼ 3.0 Å). In order to obtain structural information on superacid anions, single crystals of CsSO₃F, Cs[H(SO₃F)₂], Cs[Au(SO₃F)₄],Cs₂[Pt(SO₃F)₆],and Cs[Sb(SO₃F)₆] were prepared and their crystal structures were determined by single crystal X-ray diffraction. The structures of the superacid anions are correlated to the weak nucleophilicity of anions and hence the acid strength of the corresponding conjugate superacids. The new superacid anion [Sb(SO₃F)₆]⁻ is highly symmetric and has exceptionally short S-O and S-F bonds at the periphery of the anion, From the structure of the anion, it is suggested that [Sb(SO₃F)₆]⁻ should be very weakly nucleophilic. The corresponding conjugate superacid, the HSO₃F/Sb(SO₃F)₅ system, should be a very strong superacid system.