Squaric acid, also called quadratic acid because its four carbon atoms approximately form a square, is a diprotic organic acid with the chemical formula .[1]
The conjugate base of squaric acid is the hydrogensquarate anion ; and the conjugate base of the hydrogensquarate anion is the divalent squarate anion . 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.[2] 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 pKa1 = 1.5 for the first proton and pKa2 = 3.4 for the second is attributable to resonance stabilization of the anion.[3] 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.
Many of the reactions of squaric acid involve the OH groups. The molecule behaves similarly to a strong dicarboxylic acid. It is stronger acid than typical carboxylic acids.[4]
, pKa1 = 1.5
, pKa2 = 3.5
The OH groups are labile in squaric acid. It forms a dichloride with thionyl chloride:
The chlorides are good leaving groups, reminiscent of acid chlorides. They are displaced by diverse nucleophiles. In this way dithiosquarate can be prepared.[5]
The bis(methylether) is prepared by alkylation with trimethyl orthoformate.[6]
Dibutyl squarate is used for the treatment of warts[7] and for alopecia areata .[8]
Diethyl squarate has been used as an intermediate in the synthesis of perzinfotel.
Squaramides are prepared by displacement of alkoxy or chloride groups from (X = OR, Cl).[5] [9]
One or both of the oxygen (=O) groups in the squarate anion can be replaced by dicyanomethylene . The resulting anions, such as 1,2-bis(dicyanomethylene)squarate and 1,3-bis(dicyanomethylene)squarate, retain the aromatic character of squarate and have been called pseudo-oxocarbon anions.
Photolysis of squaric acid in a solid argon matrix at 10K affords acetylenediol.[10]
Squarate dianion behaves similarly to oxalate, forming mono- and polynuclear complexes with hard metal ions.Cobalt(II) squarate hydrate (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).[11]
Cobalt(II) squarate dihydroxide (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.[11]
Copper(II) squarate monomeric and dimeric mixed-ligand complexes were synthesized and characterized.[12] Infrared, electronic and Q-Band EPR spectra as well as magnetic susceptibilities are reported.
The same method yields iron(II) squarate dihydroxide (light brown).[11]
The original synthesis started with the ethanolysis of perfluorocyclobutene to give 1,2-diethoxy-3,3,4,4-tetrafluoro-1-cyclobutene. Hydrolysis gives the squaric acid.[13] [1]
Although impractical, squarate and related anions such as deltate and acetylenediolate are obtainable by reductive coupling of carbon monoxide using organouranium complexes.[14] [15]