UBC Theses and Dissertations
Syntheses, vibrational and solution studies of niobium(V) and tantalum(V) fluoride fluorosulfates Zhang, Dingliang
A one-step metal oxidation/ligand redistribution reaction was developed to synthesize niobium(V) and tantalum(V) fluoride fluorosulfates according to the equation: (5-n)M + nMF₅ + excess S₂0₆F₂ ⇢ 5MF[formula omitted](S0₃F)₅-[formula omitted] Five compounds, Nb₂F₉(S0₃F), NbF₄(S0₃F), NbF₃(S0₃F)₂, TaF₄(S0₃F), and TaF₃(S0₃F)₂, were isolated as colorless viscous liquids in pure form. Nb₂F₉(S0₃F) appears to be unstable, and disproportionates into solid [NbF₅]₄ and liquid NbF₄(S0₃F) on long standing. Ta₂F₉(S0₃F) cannot be synthesized in this manner. The attempted isolation of compounds with higher fluorosulfate content, MF[formula omitted](S0₃F)₅-[formula omitted] (M=Nb, Ta; n<3), was not successful. Raman and infrared spectra of the isolated compounds, MF[formula omitted](S0₃F)₅-[formula omitted] (M=Nb, Ta; n≥3), are interpreted in terms of associated structures with approximately octahedral coordination of niobium or tantalum by fluorides and fluorosulfate groups. Bidentate fluorosulfate groups, presumably bridging, are found in all these derivatives. In NbF₃(S0₃F)₂ and TaF₃(S0₃F)₂ both monodentate and bidentate fluorosulfate groups are found. Bridging fluorines are suggested for liquid Nb₂F₉(S0₉F) and, to a lesser extent, NbF₄(S0₃F) and TaF₄(S0₃F) as well. Tentative assignments of the vibrational bands are discussed. The behavior of the stable compounds, MFn(S0₃F)₅-[formula omitted] (M=Nb, Ta; n=2, 3), in HS0₃F was investigated by conductometric studies. At low Lewis acid concentrations, the results show that TaF₃(S0₃F)₂ is a weak Lewis acid in HS0₃F and that NbF₃(S0₃F)₂, NbF₄(S0₃F) and TaF₄(S0₃F) behave as very weak electrolytes and poor S0₃Fˉ acceptors. Conductivities of these solutions in HS0₃F follow the order: TaF₃(S0₃F)₂ » NbF₃(S0₃F)₂ > TaF₄(S0₃F) > NbF₄(S0₃F) . The general trend previously reported for HS0₃F/SbF₅/S0₃ and HS0₃F/AsF₅/S0₃ systems is valid for HS0₃F/NbF[formula omitted](S0₃F)₅-[formula omitted] and HS0₃F/TaF[formula omitted](S0₃F)₅-[fomula omitted] systems, i.e., the more fluorines replaced by fluorosulfate groups, the stronger the resulting superacid. At high Lewis acid concentrations, viscous and highly conducting liquids are present. For the conductivities of the neat liquids, the following order was observed: NbF₃(S0₃F)₂ > TaF₃(S0₃F)₂ > NbF₄(S0₃F) > TaF₄(S0₃F).
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