A F-19 nuclear magnetic resonance study of the conjugate Bronsted-Lewis superacid HSO3F-SbF5. Part II

Solutions of SbF5 in HSO3F with x(SbF5) = 0.012 to 0.405 are studied by 500 MHz H-1 NMR (299 K) and 471 MHz F-19 NMR (213-250 K), using NMR tubes fitted with fluoropolymer lining. The initial process during dissolution is the fast solvolysis of monomeric SbF5 in HSO3F according to SbF5 + nHSO(3)F left arrow SbF5 - n(SO3F)(n) + nHF (n = 1, 2). All HF formed during solvolysis will no longer be removed by reaction with glass, but will remain in the superacid system. Besides participation in the fast formation of various fluoro-fluorosulfato anions [SbF6 - n(SO3F)(n)](-) (n = 0, 1, 2) and acidium ions [H2X](+)((solv.)) (X= F, SO3F), HF is involved in slow-exchange reactions of the type [SbF6 - n(SO3F)(n)](-)((solv.)) + HF left arrow [SbF7 - n(SO3F)(n- 1)](-)((solv.)) + HSO3F (n = 1, 2) detected because of a delay of 3 months between sample preparation and measurements and confirmed by repeating theses measurements after a further 3 months. There are three notable differences to our earlier study, affecting concentrations of the fluoro-fluorosulfato antimonate anions observed: (i) in dilute solutions [SbF6](-) is formed in high concentrations (34.7-76.1%), with [Sb2F11](-) now clearly detected at intermediate to high SbF5 concentrations (up to 5.8%); (ii) bis-fluorosulfato anions (cis-, trans-[SbF4(SO3F)(2)](-)) are found in much lower concentrations only, which decrease further with time, while tris-fluorosulfato anions ([SbF3(SO3F)(3)](-)) are now no longer observed; (iii) these reduced concentrations of poly-fluorosulfato anions in dilute solutions are responsible for the formation of fewer mu-SO3F-oligomers at lower concentrations, when more SbF5 is added. As a consequence, the HSO3F-SbF5 magic acid system is now less complex than found previously and only seven anionic species are clearly observed.