Syntheses and Structures of Protonated Acetylenedicarboxylic Acid

Acetylenedicarboxylic acid (C4H2O4) was protonated in the binary superacidic systems HF/MF5 (M=As, Sb). Depending on the stoichiometric ratio of Lewis acid and acetylenedicarboxylic acid mono- and diprotonated acetylenedicarboxylic acid was obtained. The salts of diprotonated acetylenedicarboxylic acid were characterized by vibrational spectroscopy and in case of [C4H4O4][SbF6](2) by a single-crystal X-ray structure analysis. The salt crystallizes in the monoclinic space group C2/c with four formula units per unit cell. Furthermore, salts of monoprotonated acetylenedicarboxylic acid were synthesized and characterized by single-crystal X-ray structure analysis. The salts [C4H3O4][AsF6] and [C4H3O4][SbF6] crystallize in the monoclinic space group C2/c with four formula units per unit cell. The monoprotonation is better described as a twofold hemiprotonation of acetylenedicarboxylic acid. The experimental results are discussed together with quantum chemical calculations.

Automated summary

In this study, the protonation reaction of acetylenedicarboxylic acid and the crystal structures of its salts were investigated in the binary superacidic systems HF/MF5 (M=As, Sb). Both mono- and diprotonated acetylenedicarboxylic acid were obtained depending on the stoichiometric ratio of Lewis acid. The mono- and diprotonated salts exhibited different crystal structures, with the latter being characterized by single-crystal X-ray structure analysis. Quantum chemical calculations were also conducted to support the experimental results.