Squaric acid explained

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.

Chemical properties

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.

Derivatives

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]

Coordination complexes

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]

Synthesis

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]

See also

Notes and References

  1. Book: History of the Oxocarbons. Oxocarbons. Robert West . Robert West. 10.1016/B978-0-12-744580-9.50005-1. 1980. 1–14. Academic Press. 9780127445809.
  2. Lee . K.-S. . Kweon . J. J. . Oh . I.-H. . Lee . C. E. . 2012 . Polymorphic phase transition and thermal stability in squaric acid . J. Phys. Chem. Solids . 73 . 7. 890–895 . 10.1016/j.jpcs.2012.02.013.
  3. Robert West (chemist). Robert. West . David L.. Powell . 1963 . New Aromatic Anions. III. Molecular Orbital Calculations on Oxygenated Anions . . 85 . 17. 2577–2579 . 10.1021/ja00900a010.
  4. Web site: Acidity Tables for Heteroatom Organic Acids and Carbon Acids.
  5. Reaktionen von Quadratsäure und Quadratsäure-Derivaten. Synthesis. 1980. Arthur H. Schmidt. 1980. 12. 961. 10.1055/s-1980-29291. 101871124 .
  6. 10.15227/orgsyn.076.0189. Dimethyl Squarate and ITS Conversion to 3-Ethenyl-4-Methoxycyclobutene-1,2-Dione and 2-Butyl-6-Ethenyl-5-Methoxy-1,4-Benzoquinone. Organic Syntheses. 1999. 76. 189 . Hui . Liu. Craig S. . Tomooka. Simon L.. Xu. Benjamin R.. Yerxa. Robert W.. Sullivan. Yifeng. Xiong. Harold W.. Moore.
  7. 10.1067/mjd.2000.103631 . Squaric acid immunotherapy for warts in children . 2000 . Silverberg . Nanette B. . Lim . Joseph K. . Paller . Amy S. . Mancini . Anthony J. . Journal of the American Academy of Dermatology . 42 . 5 . 803–808 . 10775858.
  8. 10.1016/j.autrev.2016.02.021 . Modified immunotherapy for alopecia areata . 2016 . Yoshimasu . Takashi . Furukawa . Fukumi . Autoimmunity Reviews . 15 . 7 . 664–667 . 26932732.
  9. Chem. Soc. Rev. . 2011 . 40 . 2330–2346 . 10.1039/c0cs00200c . Squaramides: Physical Properties, Synthesis and Applications. Ian Storer . R. . Aciro . Caroline . Jones . Lyn H. . 5 . 21399835.
  10. Maier . Günther . Rohr . Christine . 1995 . Ethynediol: Photochemical generation and matrix-spectroscopic identification. . Liebigs Annalen . 1996 . 3. 307–309 . 10.1002/jlac.199619960303.
  11. Kumagai. Hitoshi . Sobukawa . Hideo . Kurmoo . Mohamedally . 2008 . Hydrothermal syntheses, structures and magnetic properties of coordination frameworks of divalent transition metals . Journal of Materials Science . 43 . 7. 2123–2130 . 10.1007/s10853-007-2033-8 . 2008JMatS..43.2123K . 95205908.
  12. Reinprecht, J. T.; Miller, J. G.; Vogel, G. C.; et al. (1979). "Synthesis and Characterization of Copper(II) Squarate Complexes". Inorg. Chem., 19, 927-931
  13. Hydrolysis Reactions of Halogenated Cyclobutene Ethers: Synthesis of Diketocyclobutenediol . J. D. . Park . S. . Cohen . J. R. . Lacher . amp . . 1962 . 84 . 15 . 2919–2922 . 10.1021/ja00874a015.
  14. Frey . Alistair S. . Cloke . F. Geoffrey N. . Hitchcock . Peter B. . 2008 . Mechanistic Studies on the Reductive Cyclooligomerisation of CO by U(III) Mixed Sandwich Complexes; the Molecular Structure of [(U(η-C<sub>8</sub>H<sub>6</sub>{Si′Pr<sub>3</sub>-1,4}<sub>2</sub>)(η-Cp<sup>*</sup>)]2(μ-η11-C2O2) . Journal of the American Chemical Society . 130 . 42. 13816–13817 . 10.1021/ja8059792 . 18817397.
  15. Summerscales . Owen T. . Frey . Alistair S. P. . Cloke . F. Geoffrey N. . Hitchcock . Peter B. . 2009 . Reductive disproportionation of carbon dioxide to carbonate and squarate products using a mixed-sandwich U(III) complex . Chemical Communications . 2. 198–200 . 10.1039/b815576c . 19099067.