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Schickinger, Manuel; Saal, Thomas; Zischka, Florian; Axhausen, Joachim; Stierstorfer, Karin; Morgenstern, Yvonne; Kornath, Andreas J. (2018): The Tetrahydroxydicarbenium Cation [(HO)(2)CC(OH)(2)](2+): Synthesis and Structure. In: Chemistryselect, Vol. 3, No. 43: pp. 12396-12404
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Abstract

Oxalic acid forms in superacidic solutions HF/MF5 (M = As, Sb) its corresponding mono- and diprotonated salts [HOOCC(OH)(2)][MF6] and [(HO)(2)CC(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)(2)CC(OH)(2)][SbF6](2) (4) by a single-crystal X-ray structure analysis. The salts crystallize in the monoclinic space groups P2(1)/c and P2(1)/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)(2)CC(OH)(2)](2+)center dot 4HF. In addition to this, an MEP analysis together with the NPA charges of [(OH)(2)CC(OH)(2)](2+), [(OH)(2)CC(OH)(2)](2+)center dot 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.