Synthesis and Structure of Acetylenic Acylium Cations

Acetylenedicarbonyl fluoride and propiolyl fluoride were investigated in the Lewis acidic medium consisting of SbF5/CF3CH2F. The acetylenic acyl cations are characterized by vibrational spectroscopy, NMR spectroscopy, and single-crystal X-ray diffraction. The protonated species of propiolyl fluoride decomposes in a few hours at −78 °C in a dynamic vacuum which leads to the formation of the corresponding acylium cation. The room-temperature stable methylated acyl fluoride was prepared in the ternary system CH3F/SbF5/SO2. The experimental data are discussed together with quantum chemical calculations on the M06-2X/aug-cc-pVTZ level of theory.