The Tetrahydroxydicarbenium Cation [(HO)2CC(OH)2]2+: Synthesis and Structure.

Oxalic acid forms in superacidic solutions HF/MF5 (M = As, Sb) its corresponding mono‐ and diprotonated salts [HOOCC(OH)2][MF6] and [(HO)2CC(OH)2][MF6]2 (M = As, Sb). The number of protonations is strongly dependent on the stoichiometric ratio of the Lewis acid with regard to oxalic acid. Mono‐ and diprotonated salts were characterized by vibrational spectroscopy and in the case of [HOOCC(OH)2][AsF6] (1) and [(HO)2CC(OH)2][SbF6]2 (4) by a single‐crystal X‐ray structure analysis. The salts crystallize in the monoclinic space groups P21/c and P21/n with eight, respectively four, formula units per unit cell. The vibrational spectra were compared to quantum chemical calculations of the cations [HOOCC(OH)2]+ and [(OH)2CC(OH)2]2+⋅4HF. In addition to this, an MEP analysis together with the NPA charges of [(OH)2CC(OH)2]2+, [(OH)2CC(OH)2]2+⋅6HF and oxalic acid were calculated to locate the positive charge. The protonation of oxalic acid does not lead to a remarkable change of the C‐C bond length, which is discussed for the entire series of the oxalic skeleton, starting with the dianion and ending with the tetrahydroxy dication. Tetrahydroxydicarbenium salts, containing two vicinal positive charges at the carbon atoms, were isolated for the first time. Mono‐ and diprotonated oxalic acid are characterized by vibrational spectroscopy as well as single‐crystal analyses. Surprisingly, the protonation of oxalic acid does not lead to a significant change of the CC bond length. [ABSTRACT FROM AUTHOR]