Investigations on Croconic Acid in Superacidic Media
Croconic acid reacts in superacidic solutions HF/MF5 (M = As, Sb) to yield its corresponding salts [H3O5C5][MF6] and [(H3O5C5) H(H3O5C5)][MF6](3)center dot 2HF (M = As, Sb). The degree of protonation is strongly dependent on the stoichiometric ratio of the Lewis acid regarding croconic acid. Monoprotonated salts were characterized by vibrational spectroscopy and in the case of [H3O5C5][AsF6] (1) by a single-crystal X-ray structure analysis. [H3O5C5][AsF6] crystallizes in the monoclinic space group P2(1)/c with four formula units per unit cell. The sesquiprotonated species of croconic acid [(H3O5C5)H(H3O5C5)][SbF6](3)center dot 2HF (4) was also characterized by single-crystal X-ray structure analysis. It crystallizes in the triclinic space group P (1) over bar with one formula unit per unit cell. The vibrational spectra of the monoprotonated salts were compared to quantum chemical calculations of the [H3O5C5](+)center dot 3HF cation and experimental data reported for croconic acid.