ENSIKLOPEDIA

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.^[4]

The conjugate base of squaric acid is the hydrogensquarate anion HC₄O−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. Derivatives are dyes and biochemical agents.

Chemical properties

Squaric acid is a white crystalline powder.^[5] 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 pK_a1 = 1.5 for the first proton and pK_a2 = 3.4 for the second is attributable to resonance stabilization of the anion:^[6]

Ball-and-stick model of the squarate ion

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.^[7]

C₄O₂(OH)₂ → [C₄O₃(OH)]⁻ + H⁺, pK_a1 = 1.5

[C₄O₃(OH)]⁻ → [C₄O₄]²⁻ + H⁺, pK_a2 = 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.^[8]

The bis(methylether) is prepared by alkylation with trimethyl orthoformate.^[9]

Dibutyl squarate is used for the treatment of warts^[10] and for alopecia areata .^[11]

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).^[8]^[12] 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 =C(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.^[13]

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).^[14]

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.^[14]

Copper(II) squarate monomeric and dimeric mixed-ligand complexes were synthesized and characterized.^[15] 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).^[14]

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.^[16]^[4]

Although impractical, squarate and related anions such as deltate C₃O2−3 and acetylenediolate C₂O2−2 are obtainable by reductive coupling of carbon monoxide using organouranium complexes.^[17]^[18]

References

↑ 3,4-Dihydroxy-3-cyclobutene-1,2-dione. Sigma-Aldrich
↑ "SICHERHEITSDATENBLATT". 21 March 2021.
↑ 3,4-Dihydroxy-3-cyclobutene-1,2-dione, 98+%. Alfa Aesar
1 2 Robert West (1980). "History of the Oxocarbons". In Robert West (ed.). Oxocarbons. Academic Press. pp. 1–14. doi:10.1016/B978-0-12-744580-9.50005-1. ISBN 9780127445809.
↑ Lee, K.-S.; Kweon, J. J.; Oh, I.-H.; Lee, C. E. (2012). "Polymorphic phase transition and thermal stability in squaric acid (H
₂C
₄O
₄)". J. Phys. Chem. Solids. 73 (7): 890–895. doi:10.1016/j.jpcs.2012.02.013.
↑ 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.
↑ "Acidity Tables for Heteroatom Organic Acids and Carbon Acids".
1 2 Arthur H. Schmidt (1980). "Reaktionen von Quadratsäure und Quadratsäure-Derivaten". Synthesis. 1980 (12): 961. doi:10.1055/s-1980-29291. S2CID 101871124.
↑ 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.
↑ 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.
↑ 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.
↑ 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.
↑ Maier, Günther; Rohr, Christine (1995). "Ethynediol: Photochemical generation and matrix-spectroscopic identification". Liebigs Annalen. 1996 (3): 307–309. doi:10.1002/jlac.199619960303.
1 2 3 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.
↑ 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
↑ 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.
↑ 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.
↑ 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.

Chemical properties

Derivatives

Coordination complexes

Synthesis

See also

References