Preparation and Characterization of Protonated Fumaric Acid

Fumaric acid was reacted with the binary superacidic systems HF/SbF5 and HF/AsF5. The O,O'‐diprotonated [C4H6O4]2+([MF6]–)2 (M = As, Sb) and the O‐monoprotonated [C4H5O4]+[MF6]– (M = As, Sb) species are formed depending on the stoichiometric ratio of the Lewis acid to fumaric acid. The colorless salts were characterized by low‐temperature vibrational spectroscopy. In case of the hexafluoridoantimonates single‐crystal X‐ray structure analyses were carried out. The [C4H6O4]2+([SbF6]–)2 crystallizes in the monoclinic space group C2/c with four formula units per unit cell and [C4H5O4]+[SbF6]– crystallizes in the triclinic space group P1 with one formula unit per unit cell. The protonation of fumaric acid does not cause a notable change of the C=C bond length. The experimental data are discussed together with quantum chemical calculations of the cations [C4H6O4 · 4 HF]2+ and [C4H6O4 · 2 H2CO · 2 HF]2+.