Tetramethylphosphonium fluoride: "naked" fluoride and phosphorane.

Me(4)PF 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 Me(4)P+ cations forming six H...F contacts between 218 and 240 pm. The compound is stable below 120 degrees 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 Me(4)PF molecule has a trigonal bipyramidal structure with one methyl group and the fluorine atom in axial positions and bond lengths of d(PC(eq)) = 182.6(4) pm, d(PC(ax)) = 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 (1)H, (13)C, (19)F, and (31)P NMR spectroscopy. The hygroscopic Me(4)PF forms a tetrahydrate which crystallizes in the space group I4(1)/a with a = 1106.1(1) pm, c = 816.3(1) pm, and Z = 4. The fluoride ion in Me(4)PF.4 H(2)O is surrounded by four water molecules. These units form a three-dimensional network in which the Me(4)P+ cations are embedded without any contacts.