Investigations on Squaric Acid in Superacidic Media

Abstract

The syntheses of [OC(COX) ][MF ] and [(COX) ][MF ] ⋅2 HF were carried out in superacidic media XF/MF (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) ][MF ] ⋅2 HF (M=As, Sb), by single-crystal X-ray structure analyses. [(COH) ][AsF ] crystallizes in the monoclinic space group P2 /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 HF on the PBE1PBE/6-311G++(3df,3pd) level of theory. Furthermore, a salt of monoprotonated squaric acid, [OC(COH) ][AsF ], was characterized by a single-crystal X-ray structure analysis. It crystallizes in the monoclinic space group P2 /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.

Syntheses and Structures of Protonated Acetylenedicarboxylic Acid

“…To address this issue in the calculation, two HF molecules were added to simulate anion‐cation contacts and two formaldehyde molecules were added to simulate the interactions of the cationic chain in the solid state (Figure 7). This approach has already been successfully applied to simulate hydrogen bonding and is in good accordance with the data from the crystal structures ( 3 ) and ( 4 ) [11,18] …”

“…This approach has already been successfully applied to simulate hydrogen bonding and is in good accordance with the data from the crystal structures (3) and ( 4). [11,18] Comparing the crystal and calculated gas phase structures, it stands out that the carbon scaffold is not changed significantly by the degree of protonation. Therefore, it is interesting to compare the protonated species of this work with anionic species of acetylenedicarboxylic acid from the literature.…”

A concise review on application of squaric acid derivatives and their metal complexes in medicine

“…It is a strong hydrogen-bonded and remarkably stable solid and forms clear crystals. [15][16][17][18] Squaric acid has a four-membered cyclic ring that exhibits two acidic hydroxyl groups, as well as two highly polarized carbonyl groups. [18][19][20] Squaric acid can form mono-and dianions (Figure 3).…”

Dimorpholinoacetylene and Its Use for the Synthesis of Tetraaminocyclobutadiene Species

“…In case of C 4 Ph 4 2+ , C 4 F 4 2+ and C 4 (CN) 4 2+ , however, ring planar minimum geometries (dihedral angle of 0°) were theoretically predicted due to possible delocalisation of the positive charge on the substituents [26] . Accordingly, planar ( C 4h ) geometries were recently confirmed for the dications in the salts [C 4 (OH) 4 ][MF 6 ] 2 ⋅ 2HF (M=As, Sb) by single‐crystal X‐ray structure analyses, [27] which is in line with earlier predictions for diprotonated squaric acid C 4 (OH) 4 2+ [28] . A planar geometry was also predicted for the tetraaminocyclobutadiene dication, C 4 (NH 2 ) 4 2+ , based on semi‐empirical quantum chemical calculations, [29] in contrast to the experimentally observed slightly puckered structure established for the tetrapiperidino derivative IV [11] …”