Halogenated carboxonium salts: Preparation and structural analysis

We have studied the preparation and isolation of the halogenated alkylidene oxonium salts CH3OCHX+MF6- (X = Cl, F; M = As, Sb). Addition of dichlorome thylmethylether at -78 degrees C to the superacidic media HF/MF5 resulted i n the formation of the chlorine compounds CH3OCHCl+MF6-, whereas at -65 deg rees C the fluorinated salts CH3OCHF+MF6- are formed by a chlorine/fluorine exchange. The salts are characterized by their low-temperature IR and Rama n spectra. Additionally, the crystal structures of the hexafluoroantimonate s are presented. The structures of the cations turned out to be different i n some points. The salt CH3OCHF+SbF6- crystallizes in the monoclinic space group P2(1)/c with 4 molecules in the unit cell and a = 9.942(2), b = 7.454 (2), and c = 10.297(3) Angstrom; beta = 111.27(2)degrees. The salt CH3OCHCl +SbF6- crystallizes in the monoclinic space group P2(1)/c with 4 formula in the unit cell and a = 5.970(3), b = 12.019(5), and c = 10.994(5) Angstrom; beta = 92.59(3)degrees.