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 SO2. 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. The ability of the strong Lewis acid SbF5 to act as a fluoride‐ion donor has been demonstrated for the first time through the synthesis of donor‐stabilised [SbF4]+ cations. In the presence of bidentate N‐donor ligands, SbF5 undergoes ligand‐induced autoionisation to yield robust [SbF4(L−L)]+ (L−L=2,2’bipyridine or 1,10‐phenanthroline) salts. DFT calculations and NBO analyses show strong Sb−N interactions that are necessary to stabilise the [SbF4]+ cation.