Tetramethylphosphonium Fluoride: “Naked” Fluoride and Phosphorane

Me4PF was investigated in the solid state, in the gas phase, and in solutions. Vibrational spectra of the solid and
a single-crystal structure show an ionic tetramethylphosphonium fluoride. The compound crystallizes in the space
group Pbca with a = 1016.0(1), b = 1018.0(1), c = 1205.8(4) pm, and Z = 8. The fluoride ion is nearly trigonal
planar surrounded by three Me4P+ cations forming six H···F contacts between 218 and 240 pm. The compound
is stable below 120 °C and sublimes in a vacuum. It possesses a phosphorane structure in the gas phase that was
studied by electron diffraction and vibrational spectra, and additionally by theoretical calculations. The Me4PF molecule
has a trigonal bipyramidal structure with one methyl group and the fluorine atom in axial positions and bond
lengths of d(PCeq) = 182.6(4) pm, d(PCax) = 188.4(8) pm, and d(PF) = 175.3(6) pm. The compound is remarkably
soluble in acetonitrile, water, and alcohols, and slightly soluble in benzene, dimethyl ether, and diethyl ether. The
solutions were studied by 1H, 13C, 19F, and 31P NMR spectroscopy. The hygroscopic Me4PF forms a tetrahydrate
which crytallizes in the space group I41/a with a = 1106.1(1) pm, c = 816.3(1) pm, and Z = 4. The fluoride ion
in Me4PF·4 H2O is surrounded by four water molecules. These units form a three-dimensional network in which the
Me4P+ cations are embedded without any contacts.