Investigations on Squaric Acid in Superacidic Media

The syntheses of [OC(COX)₃ ][MF₆ ] and [(COX)₄ ][MF₆ ]₂ ⋅2 HF were carried out in superacidic media XF/MF₅ (M=As, Sb; X=H, D). The degree of protonation is highly dependent on the stoichiometric ratio of the Lewis acid with regard to squaric acid. The salts of diprotonated squaric acid were characterized by Raman spectroscopy and, in the case of [(COH)₄ ][MF₆ ]₂ ⋅2 HF (M=As, Sb), by single-crystal X-ray structure analyses. [(COH)₄ ][AsF₆ ] crystallizes in the monoclinic space group P2₁ /n with two formula units per unit cell. Analysis of the vibrational spectra was achieved with the support of quantum chemical calculations of the cation [(COH)₄ ]²⁺ ⋅4 HF on the PBE1PBE/6-311G++(3df,3pd) level of theory. Furthermore, a salt of monoprotonated squaric acid, [OC(COH)₃ ][AsF₆ ], was characterized by a single-crystal X-ray structure analysis. It crystallizes in the monoclinic space group P2₁ /n with four formula units per unit cell. The protonation of squaric acid leads to a change of the carbon skeleton, which is discussed for the entire series, starting with the dianion of squaric acid and ending with the tetrahydroxy dication.