Squaric acid is a white crystalline powder with a thermal decomposition point of 245 °C at ambient pressure. The onset of thermal decomposition depends on the different thermodynamic conditions such as heating rates. The structure of squaric acid is not a perfect square, as the carbon–carbon bond lengths are not quite equal. The high acidity with pKa = 1.5 for the first proton and pKa = 3.4 for the second is attributable to resonance stabilization of the anion. Because the negative charges are equally distributed between each oxygen atom, the dianion of squaric acid is completely symmetrical with all C−C bond lengths identical and all C−O bond lengths identical. Another, quantum mechanical, way of describing the dianion is to assume that the π electrons of the two C=O double bonds are shifted to the oxygen atoms, so that all four oxygens become single-bonded −O− groups and a double positiveelectric charge is left in the ring of carbon atoms. In this way the ring fits Hückel's rule for aromaticity. The total symmetry of the dianion is a consequence of charge distribution and aromaticity.
Derivatives
of squaric acid in a solid argon matrix at affords acetylenediol. Cobalt squarate hydrate Co2 can be prepared by autoclavingcobalt hydroxide and squaric acid in water at 200 °C. The water is bound to the cobalt atom, and the crystal structure consists of a cubic arrangement of hollow cells, whose walls are either six squarate anions or several water molecules. Cobalt squarate dihydroxide Co322·3H2O is obtained together with the previous compound. It has a columnar structure including channels filled with water molecules; these can be removed and replaced without destroying the crystal structure. The chains are ferromagnetic; they are coupled antiferromagnetically in the hydrated form, ferromagnetically in the anhydrous form. Copper squarate monomeric and dimeric mixed-ligand complexes were synthesized and characterized. Infrared, electronic and Q-Band EPR spectra as well as magnetic susceptibilities are reported. The same method yields iron squarate dihydroxide Fe22. One or both of the oxygen groups in the squarate anion can be replaced by other chalcogenides such as sulfur or other divalent groups, such as dicyanomethylene =C2. The resulting anions, such as 1,2-bissquarate and 1,3-bissquarate, retain the aromatic character of squarate and have been called pseudo-oxocarbon anions. There have been theoretical investigations of the analogous compound obtained by substituting amino groups for the hydroxyl groups to yield 1,2-diamino-3-cyclobutenedione, and of a compound consisting of two squarate rings bridged by bonds to form bispiperazine. Squaramides are amides of squaric acids. They are four membered vinylogously conjugated diamides. Hydrogen bonding occurs at four locations and is aided by delocalisation of electrons on nitrogen's orbitals to the carbonyl group. Hydrogen bonding and thus delocalization further increases ring aromaticity.
Medically, SADBE or squaric acid dibutyl ester or dibutyl squarate derives from a squaric acid. is used for the treatment of warts. Squaric acid dibutyl ester is also used for treating alopecia areata or alopecia totalis through topical immunotherapy involving the production of an allergic rash. Squaric acid dibutylester is currently undergoing trials for use in treating herpes labialis. Diethylsquarate was utilized in the synthesis of Perzinfotel.
