Squaric acid
Watchedfieldschanged
Verifiedrevid470470338
Reference[1]
Pin3,4-Dihydroxycyclobut-3-ene-1,2-dione
OthernamesQuadratic acid
Cyclobutenedioic acid

Squaric acid or quadratic acid (so named because its four carbon atoms approximately form a square) is a diprotic organic acid with the chemical formula C4O2(OH)2.[2]

The conjugate base of squaric acid is the hydrogensquarate anion HC4O4−; and the conjugate base of the hydrogensquarate anion is the divalent squarate anion C4O4(2−). This is one of the oxocarbon anions, which consist only of carbon and oxygen.

Squaric acid is a reagent for chemical synthesis. Derivatives are dyes and biochemical agents.

Chemical properties

Squaric acid is a white crystalline powder.[3] 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:[4]

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.[5]

C4O2(OH)2 → [C4O3(OH)]− + H+, pKa1 = 1.5
[C4O3(OH)]− → [C4O4](2−) + H+, pKa2 = 3.5

The OH groups are labile, and squaric acid forms a dichloride with thionyl chloride:

C4O2(OH)2 + 2 SOCl2 → C4O2Cl2 + 2 HCl + 2 SO2

The chlorides leave easily, reminiscent of acid chlorides. They are displaced by diverse nucleophiles. In this way dithiosquarate can be prepared.[6]

The bis(methylether) is prepared by alkylation with trimethyl orthoformate.[7]

Dibutyl squarate is used for the treatment of warts[8] and for alopecia areata.[9]

Diethyl squarate has been used as an intermediate in the synthesis of perzinfotel.[citation needed]

Squaramides are prepared by displacement of alkoxy or chloride groups from C4O2X2 (X = OR, Cl).[6][10] Likewise, reaction with arenes gives photosensitive squaraine dyes.

Other derivatives inhibit of protein tyrosine phosphatases.

One or both of the oxygen (=O) groups in the squarate anion can be replaced by dicyanomethylene \dC(CN)2. 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 10 K (−263 °C) affords acetylenediol.[11]

Coordination complexes

The squarate dianion behaves similarly to oxalate, forming mono- and polynuclear complexes with hard metal ions. Cobalt(II) squarate hydrate Co(C4O4)*2H2O (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).[12]

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.[12]

Copper(II) squarate monomeric and dimeric mixed-ligand complexes were synthesized and characterized.[13] Infrared, electronic and Q-Band EPR spectra as well as magnetic susceptibilities are reported.

The same method yields iron(II) squarate dihydroxide Fe2(OH)2(C4O4) (light brown).[12]

Synthesis

The original synthesis started with the ethanolysis of hexafluorocyclobutene to give 1,2-diethoxy-3,3,4,4-tetrafluoro-1-cyclobutene. Hydrolysis gives squaric acid.[14][2]

Although impractical, squarate and related anions such as deltate C3O3(2-) and acetylenediolate C2O2(2-) are obtainable by reductive coupling of carbon monoxide using organouranium complexes.[15][16]

See also

References

  1. ^ 3,4-Dihydroxy-3-cyclobutene-1,2-dione. Sigma-Aldrich
  2. ^ Robert West (1980). "History of the Oxocarbons". Oxocarbons. Robert West (ed.). Academic Press. pp. 1–14. doi:10.1016/B978-0-12-744580-9.50005-1. ISBN 9780127445809.
  3. ^ Lee, K.-S.; Kweon, J. J.; Oh, I.-H.; Lee, C. E. (2012). "Polymorphic phase transition and thermal stability in squaric acid (H2C4O4)". J. Phys. Chem. Solids. 73 (7): 890–895. doi:10.1016/j.jpcs.2012.02.013
  4. ^ West, Robert & Powell, David L. (1963). "New Aromatic Anions. III. Molecular Orbital Calculations on Oxygenated Anions". J. Am. Chem. Soc.. 85 (17): 2577–2579. doi:10.1021/ja00900a010
  5. ^ "Acidity Tables for Heteroatom Organic Acids and Carbon Acids"
  6. ^ Arthur H. Schmidt (1980). "Reaktionen von Quadratsäure und Quadratsäure-Derivaten". Synthesis. 1980 (12): 961. doi:10.1055/s-1980-29291. S2CID 101871124
  7. ^ Liu, Hui; Tomooka, Craig S.; Xu, Simon L.; Yerxa, Benjamin R.; Sullivan, Robert W.; Xiong, Yifeng; Moore, Harold W. (1999). "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. 76: 189. doi:10.15227/orgsyn.076.0189
  8. ^ Silverberg, Nanette B.; Lim, Joseph K.; Paller, Amy S.; Mancini, Anthony J. (2000). "Squaric acid immunotherapy for warts in children". Journal of the American Academy of Dermatology. 42 (5): 803–808. doi:10.1067/mjd.2000.103631. PMID 10775858
  9. ^ Yoshimasu, Takashi & Furukawa, Fukumi (2016). "Modified immunotherapy for alopecia areata". Autoimmunity Reviews. 15 (7): 664–667. doi:10.1016/j.autrev.2016.02.021. PMID 26932732
  10. ^ Ian Storer, R.; Aciro, Caroline; Jones, Lyn H. (2011). "Squaramides: Physical Properties, Synthesis and Applications". Chem. Soc. Rev.. 40 (5): 2330–2346. doi:10.1039/c0cs00200c. PMID 21399835
  11. ^ Maier, Günther & Rohr, Christine (1995). "Ethynediol: Photochemical generation and matrix-spectroscopic identification.". Liebigs Annalen. 1996 (3): 307–309. doi:10.1002/jlac.199619960303
  12. ^ Hitoshi, Kumagai; Hideo, Sobukawa; Mohamedally, Kurmoo (2008). "Hydrothermal syntheses, structures and magnetic properties of coordination frameworks of divalent transition metals". Journal of Materials Science. 43 (7): 2123–2130. Bibcode:2008JMatS..43.2123K. doi:10.1007/s10853-007-2033-8. S2CID 95205908
  13. ^ 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
  14. ^ Park, J. D.; Cohen, S.; & Lacher, J. R. (1962). "Hydrolysis Reactions of Halogenated Cyclobutene Ethers: Synthesis of Diketocyclobutenediol". J. Am. Chem. Soc.. 84 (15): 2919–2922. doi:10.1021/ja00874a015
  15. ^ 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-C8H6{Si′Pr3-1,4}2)(η-Cp*)]2(μ-η1:η1-C2O2)". Journal of the American Chemical Society. 130 (42): 13816–13817. doi:10.1021/ja8059792. PMID 18817397
  16. ^ 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. doi:10.1039/b815576c. PMID 19099067