Donor-Stabilised [SbF4]+: SbF5 as a Fluoride-Ion Donor

Antimony pentafluoride is a strong Lewis acid and fluoride-ion acceptor that has not previously demonstrated any discreet fluoride-ion donor properties. The first donor-stabilised SbF₄]⁺ cations were prepared from the autoionisation of SbF₅ in the presence of bidentate N-donor ligands 2,2’-bipyridine (bipy) and 1,10-phenanthroline (phen) as their SbF₆]^? salts. The SbF₄(N?N)]Sb₂F₁₁] (N?N=bipy, phen) salts were synthesised by the addition of one equivalent of SbF₅?SO₂ to SbF₄(N?N)]SbF₆] in liquid SO_2. The salts show remarkable stability and were characterised by Raman spectroscopy and multinuclear NMR spectroscopy. The crystal structures of SbF₄(phen)]SbF₆] ? 3CH₃CN and SbF₄(phen)]SbF₆] ? 2SO₂ were determined, showing distorted octahedral cations. DFT calculations and NBO analyses reveal that significant degree of electron-pair donation from N to Sb stabilizes SbF₄]⁺ with the Sb?N bond strength being approximately two thirds of that of the Sb?F bonds in these cations and the cationic charge being primarily ligand-centred.