Strengthening of the C-F Bond in Fumaryl Fluoride with Superacids

The reaction of fumaryl fluoride with the superacidic solutions XF/MF₅ (X=H, D; M=As, Sb) results in the formation of the monoprotonated and diprotonated species, dependent on the stoichiometric ratio of the Lewis acid to fumaryl fluoride. The salts [C₄ H₃ F₂ O₂ ]⁺ [MF₆ ]^- (M=As, Sb) and [C₄ H₂ X₂ F₂ O₂ ]²⁺ ([MF₆ ]^- )₂ (X=H, D; M=As, Sb) are the first examples with a protonated acyl fluoride moiety. They were characterized by low-temperature vibrational spectroscopy. Low-temperature NMR spectroscopy and single-crystal X-ray structure analyses were carried out for [C₄ H₃ F₂ O₂ ]⁺ [SbF₆ ]^- as well as for [C₄ H₄ F₂ O₂ ]²⁺ ([MF₆ ]^- )₂ (M=As, Sb). The experimental results are discussed together with quantum chemical calculations of the cations [C₄ H₄ F₂ O₂  ⋅ 2 HF]²⁺ and [C₄ H₃ F₂ O₂  ⋅ HF]⁺ at the B3LYP/aug-cc-pVTZ level of theory. In addition, electrostatic potential (ESP) maps combined with natural population analysis (NPA) charges were calculated in order to investigate the electron distribution and the charge-related properties of the diprotonated species. The C-F bond lengths in the protonated dication are considerably reduced on account of the +R effect.