Squaric acid

Squaric acid, also called quadratic acid because its four carbon atoms approximately form a square, is a dibasic organic acid with the chemical formula C4H2O4. Squaric acid, also called quadratic acid because its four carbon atoms approximately form a square, is a dibasic organic acid with the chemical formula C4H2O4. The conjugate base of squaric acid is the hydrogensquarate anion C4HO−4; and the conjugate base of the hydrogensquarate anion is the divalent squarate anion C4O2−4. This is one of the oxocarbon anions, which consist only of carbon and oxygen. Squaric acid is a reagent for chemical synthesis, used for instance to make photosensitive squaraine dyes and inhibitors of protein tyrosine phosphatases. 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 (unlike squaric acid itself) 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 positive electric charge is left in the ring of carbon atoms. In this way the ring fits Hückel's rule for aromaticity (2 π electrons = 4n + 2 with n = 0). The total symmetry of the dianion is a consequence of charge distribution and aromaticity. Photolysis of squaric acid in a solid argon matrix at 10 K (−263 °C) affords acetylenediol. Cobalt(II) squarate hydrate Co(C4O4)(H2O)2 (yellow, cubic) can be prepared by autoclaving cobalt(II) 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 (leaving a 7 Å wide void) or several water molecules (leaving a 5 Å void). Cobalt(II) squarate dihydroxide Co3(OH)2(C4O4)2·3H2O (brown) 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.