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Fluorosulfate derivatives of niobium and tantalum and their behavior as components of novel superacid systems Cicha, Walter Vladimir


The goal of this study was to develop new superacid systems based on fluorosulfuric acid, HSO₃F, (the strongest monoprotonic Bronsted acid) and metal fluorosulfates capable of acting as Lewis acids. The in situ oxidation of niobium and tantalum in HSO₃F by bis(fluorosulfuryl) peroxide, S₂O₆F₂ resulted in the formation of the highly solvated Lewis acids M(SO₃F)₅ with M = Nb or Ta. Based on electrical conductivity measurements, both solutes were found to behave as moderately strong, monoprotonic acids in HSO₃F, with Ta(SO₃F)₅ the markedly stronger acid of the two. The Hammett Acidity Function, H₀, determined for the HSO₃F-Ta(SO₃F)₅ superacid system confirmed its high acidity, which clearly exceeds that of HSO₃F-SDF₅ ("Magic Acid"), the most frequently used superacid system. In addition, both the solubility and acidity in HSO₃F of the two new Lewis acids are vastly greater than those of the analogous fluorides, NbF₅ and TaF₅, in either HF or HSO₃F. The high solubility of Nb(SO₃F)₅ and Ta(SO₃F)₅ allowed the study of their solution behavior, using ¹H, ¹⁹F and ⁹³Nb NMR, as well as Raman spectroscopy. Evidence for the existence of M(SO₃F)₅, with M = Nb or Ta, comes from the synthesis of the salts Mx'[M(SO₃F)[sub 5+x]], with M' = Cs or Ba and x = 1 or 2, which were characterized by vibrational spectroscopy. Salts with anions of the types [M(SO₃F)₆] ⁻ or [M(SO₃F)₇]²⁻ have previously not been isolated. In solution, multicomponent equilibria appeared to be present between the anions [M(SO₃F)₆] ⁻ and [M(SO₃F)₇]²⁻ with M = Nb or Ta. Synthesis of TaF₄ (SO₃F) from a 4:1 mixture of TaF₅ and Ta(SO₃F)₅ in HSO₃F as well as the formation of NbF₂ (SO₃F)₃ from a concentrated solution of HSO₃F-Nb(SO₃F)₅ suggested the possibuity of a new family of superacid systems of the type HSO₃F-MF[sub x](SO₃F)[sub 5-x], with x = 1-4. Initial investigations are reported. In addition, preliminary work dealing with analogous trifluoromethyl sulfuric acid (HSO₃CF₃) systems is also discussed. During the course of this study, bis(fluorosulfuryl) peroxide (S₂O₆F₂) was found to behave as a weak base soluble only in acids stronger than 100% sulfuric acid. Addition of potassium fluorosulfate, KSO₃F, to reduce the acidity of HSO₃F also lowered the acid's ability to dissolve S₂O₆F₂. The HSO₃F- S₂O₆F₂ system was studied using Raman, IH and WF NMR, and ESR spectroscopy, which led to evidence for proton transfer, hydrogen-bridging and fluorosulfate exchange between the solvent (HSO₃F) and solute (S₂O₆F₂).

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