Crystallization from hydrochloric acid affords the solid-state structure of croconic acid (175 years after its discovery) and a novel hydrogen-bonded network

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Abstract

The structure of croconic acid is determined on single crystals grown from a solution of HCl 1 M; it is shown that O–H⋯O interactions organize the H₂C₅O₅ molecules in an ‘accordion’ type supramolecular arrangement of hydrogen-bonded molecules; the stacking of [Cl]⁻·H₂C₅O₅ units in the organometallic co-crystal [(η⁵-C₅H₅)₂Co][Cl]·H₂C₅O₅ is also analysed.

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Introduction

Gmelin reported in 1825 the preparation of the ion C₅O₅²⁻ and of the corresponding acid H₂C₅O₅. This acid he called croconic acid because of its yellow colour.¹ The dianion C₅O₅²⁻ belongs to the family of oxocarbon anions that include the deltate, squarate, and rhodizonate anions.² The interest in these anions was originated by the view that cyclic oxocarbon anions of the general formula C_nO_n²⁻ might show aromaticity stabilised by electron delocalisation of the π electrons around the ring.³ It is worth recalling that the isolation by Gmelin of croconic acid and of the croconate dianion is contemporary to the isolation by Faraday of benzene, the prototype of aromatic molecules. While the solid state structures of deltic⁴ and squaric acid⁵ were established a long time ago, those of the heavier members remain unknown. What could be the reason for this ‘absence’ from the vast repertoire of organic molecules characterised by diffraction methods⁶ is hard to say. The crystallisation of croconic acid by us is the result of our systematic quest for new building blocks to be utilised in the crystal engineering⁷ of hybrid organic–organometallic materials.⁸

Results and discussion

Yellow, transparent, single crystals of croconic acid 1 were obtained from crystallisation of croconic acid from an aqueous solution of HCl 1 M.⁹ The co-crystal [(η⁵-C₅H₅)₂Co][Cl]·H₂C₅O₅2 was obtained via reaction of rhodizonic acid H₂C₆O₆ with the hydroxide [(η⁵-C₅H₅)₂Co][OH], in order to attain acid ring contraction and formation of the croconate dianion,¹⁰ followed by acidification of the solution with HCl.⁹ The structures of 1 and 2 were determined by single crystal X-ray diffraction (Table 1). Both structures possess some remarkable features which will be summarised in the following.

Table 1 Single crystal X-ray diffraction data for 1 and 2^a

Properties	1 ^b	2 ^c
Formula	C2H2O5	C15H12CoClO5
a Click b100020i.txt for full crystallographic data (CCDC nos. 147324–147325). b The hydrogen atoms in 1, directly located from the Fourier maps, are responsible for the reduction of symmetry from space group Pbca to Pca21. Data were collected on a Bruker AXS SMART diffractometer. c Data collected on a Nonius CAD4 diffractometer equipped with an Oxford Cryostream liquid N2 device. Both diffractometers equipped with a graphite monochromator (Mo-Kα radiation, λ⊕=⊕0.71073 Å). SHELXS-97^11 and SHELXL97^11 were used for structure solution and refinement based on F^2. SCHAKAL97^12was used for the graphical representation of the results.
T/K	273(2)	223(2)
M	142.07	366.63
Crystal system	Orthorhombic	Triclinic
Space group	Pca21	P1̄
a/Å	8.7108(8)	6.807(3)
b/Å	5.1683(5)	10.787(3)
c/Å	10.9562(9)	11.629(7)
α/°	90	67.52(4)
β/°	90	73.26(5)
γ/°	90	87.84(5)
V/Å^3	493.25(8)	753.0(6)
Z	4	2
F(000)	288	372
μ/mm^−1	0.179	1.336
θ/°	4–34	3–30
Number of reflections (independent)	6525 (1838)	4581 (4381)
Refinement on F^2 (number of parameters)	91	181
wR (F^2, all refls.)	0.1240	0.2035
R 1 (I ⊕>⊕2σ(I))	0.0549	0.0586

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Fig. 1a shows the hydrogen-bonded network in crystalline 1. Since hydrogen atom positions could be observed from the Fourier maps it is possible to examine the hydrogen bond patterns in some detail. Each molecule of croconic acid is linked to four molecules by two pairs of hydrogen bonds forming sheets of large tetrameric rings. What makes this structure rather unusual is, however, the way the hydrogen-bonded sheets are arranged in three dimensions. Fig. 1c shows how the sheets are pleated in an ‘accordion’ pattern and shifted to avoid overlap between molecules in different layers. The angle between croconic acid planes is 69.90 deg with the hinges constituted of O–H⋯O units as shown in Fig. 1b. It is interesting to note that squaric acid also forms large tetramolecular rings in the solid state, but the sheets are flat with each molecule of squaric acid placed above and below the centres of the tetramolecular rings. The hydrogen bonds in 1 [O(H)⋯O 2.628(5), 2.617(5) Å] are slightly longer than in squaric acid [O(H)⋯O 2.532(4), 2.544(4) Å].

Fig. 1 a) A pleated sheet, extending in the ac-plane, formed by hydrogen-bonded molecules of croconic acid in crystalline 1. Click image or fig1.htm to access a 3D representation. b) The hinge of the ‘accordion’ is defined by O–H⋯O bonds. c) Space-filling representation down the c-axis of the ‘accordion’ type arrangement adopted by the croconic acid sheets.

Crystalline 2 was obtained in an attempt to intercalate croconic acid moieties between the cyclopentadienyl rings of the cobalticinium cation. The idea stemmed from our recently reported success in intercalating hydrogen squarate anions between [(η⁵-C₅H₅)₂Co]⁺ and [(η⁶-C₆H₆)₂Cr]⁺ cations.¹³ Since direct utilisation of croconic acid failed to give the desired products, we attempted in situ preparation of the hydrogen croconate anion by exploiting the well established rhodizonate ring contraction upon treatment with a base, presently the [(η⁵-C₅H₅)₂Co][OH] hydroxide. The addition of HCl to the C₅O₅²⁻ anion thus formed, however, results in a strong interaction of the chloride anion with croconic acid, leading to crystallisation of [(η⁵-C₅H₅)₂Co][Cl]·H₂C₅O₅. Fig. 2a is a space-filling representation of how the acid H₂C₅O₅ chelates the chloride anion by means of a twin O–H⋯Cl hydrogen bond [O(H)–Cl⁻ distances 2.986(5) and 2.939(5) Å]. In addition to this, the whole [Cl]⁻·H₂C₅O₅ system is encapsulated within a system of six cobalticinium cations, which interact via C–H⋯O interactions [(C)H⋯O distances shorter than 2.4 Å: 2.195(6), 2.182(6) Å, 2.347(6) and 2.376(6) Å] with the available O-atoms’ lone-pairs on the acid molecules (see Fig. 2a). The anions alternate along the pile resulting in a sort of interdigitation of croconic acid molecules between chloride anions as shown in Fig. 2b. The distance between [Cl]⁻·H₂C₅O₅ planes is ca. 3.3 Å. Finally, it is worth noting that the croconic acid molecule interacting with the chloride anion in 2 shows syn arrangement of the protons while the conformation is anti in crystalline 1 and differs slightly, but appreciably, in terms of molecular geometry in the two crystalline forms, as shown in Table 2. These differences should be ascribed to the different intermolecular interactions established by the croconic acid in crystalline 1 and 2. The difference reflects the different intermolecular interactions involving the OH groups in the two systems.

Table 2 Relevant structural parameters for the croconic acid molecules in 1 and 2 (Å, deg)

	1	2
a Distances based on observed hydrogen atom positions.
C1–O1	1.295(4)	1.329(8)
C2–O2	1.306(4)	1.325(8)
C3–O3	1.233(5)	1.206(8)
C4–O4	1.213(4)	1.175(10)
C5–O5	1.205(2)	1.208(8)
C1–C2	1.382(5)	1.364(9)
C2–C3	1.442(3)	1.435(10)
C3–C4	1.508(6)	1.520(11)
C4–C5	1.518(6)	1.515(11)
C5–C1	1.466(6)	1.453(10)
O1–H1^a	0.933	0.889
O2–H2^a	1.002	0.995
O1–C1–C2	122.1(4)	130.6(7)
O2–C2–C1	128.9(4)	129.4(7)

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Fig. 2 a) Space-filling representation of crystalline 2 along the a axis, showing the chelation of the Cl⁻ anion (green balls) by the croconic acid. b) A side-view of the [Cl]⁻·H₂C₅O₅ pile surrounded by the cobalticinium cations. H_(cp) omitted for clarity. Click image or fig2.htm to access a 3D representation.

Summary

The quest for new synthons¹⁴ to be utilised in crystal engineering has afforded some new insights into a piece of structural chemistry that goes back to the early days of organic chemistry. Crystallisation of croconic acid alone or in its co-crystal 2 has been possible only from acidic solutions.

Acknowledgements

We thank M. U. R. S. T. (projects Supramolecular Devices 1999–2000 and Solid Supermolecules 2000–2001) and the Universities of Bologna (project Innovative Materials) and Sassari for financial support.

Notes and references

1. ‘Von τó χρóχoν, der Safran, oder das Eigelb, wegen der gelben und rothgelben Farbe der Krokonsäure und vieler ihrer Verbindungen. Sollte es sich bestätigen, daß sie eine Wasserstoffsäure ist, so wäre sie Hydrokrokonsäure zu nennen und ihr Radical: Krokon.’ (a) L. Gmelin, Ann. Phys. Chem., 1825, 4, 31 (from the Greek τó χρóχoν, saffron or yalk, because of the yellow and orange colour of the croconic acid and of many of its compounds. Were it confirmed that it is a hydroxy acid, then it should be given the name of hydrocroconic acid and its radical should be called crocon)..
2. (a) Oxocarbons, ed. R. West, Academic Press, New York, 1980.; (b) F. Serratosa, Acc. Chem. Res., 1983, 16, 170; (c) P. R. Schleyer, K. Najafian, B. Kiran and H. Jiao, J. Org. Chem., 2000, 65, 426.
3. (a) R. West and D. L. Powell, J. Am. Chem. Soc., 1963, 82, 6204; (b) J. Aihara, J. Am. Chem. Soc., 1981, 103, 1633.
4. D. Semmingsen and P. Groth, J. Am. Chem. Soc., 1987, 109, 7238.
5. Y. Wang, G. D. Stucky and J. M. Williams, J. Chem. Soc., Perkin Trans. 2, 1974, 35, 2.
6. The number of organic compounds in the April 2000 version of the Cambridge Structural Database is ca. 97 000..
7. (a) Crystal Engineering: from Molecules and Crystals to Materials, ed. D. Braga, F. Grepioni and A. G. Orpen, Kluwer Academic Publishers, Dordrecht, 1999.; (b) D. Braga, F. Grepioni and G. R. Desiraju, Chem Rev., 1998, 98, 1375; (c) See also the perspective article in the Dalton Discussion Inorganic Crystal Engineering, S. Mann, J. Chem. Soc., Dalton Trans., 2000, 3753 and references therein.
8. (a) D. Braga, C. Bazzi, L. Maini and F. Grepioni, CrystEngComm, 1999, 5; (b) D. Braga and F. Grepioni, J. Chem. Soc., Dalton Trans., 1999, 1; (c) D. Braga, L. Maini, F. Grepioni, A. DeCian, O. Felix, J. Fisher and M. W. Hosseini, New. J. Chem., 2000, 24, 547; (d) D. Braga and F. Grepioni, Acc. Chem. Res., 2000, 33, 601.
9. 1: Croconic acid purchased from Aldrich was dissolved in HCl 1 M. Yellow crystals suitable for X-ray diffraction were obtained from slow evaporation of the solution. 2: 100 mg (0.53 mmol) of cobaltocene were oxidised in 15 ml of water in air, then 107 mg (0.53 mmol) of rhodizonic acid were added under stirring; after 3 h HCl 1 M was added dropwise until pH⊕=⊕1 was reached. Crystals suitable for X-ray diffraction were obtained by slow evaporation of the solution..
10. R. Nietzki, Ber. Dtsch. Chem. Ges., 1887, 20, 1617.
11. G. M. Sheldrick, SHELXL97, Program for Crystal Structure Determination; University of Göttingen: Göttingen, Germany, 1997..
12. E. Keller, SCHAKAL97 Graphical Representation of Molecular Models; University of Freiburg, Germany, 1997..
13. D. Braga, L. Maini, L. Prodi, A. Caneschi, R. Sessoli and F. Grepioni, Chem. Eur. J., 2000, 6, 1310.
14. G. R. Desiraju, Angew. Chem., Int. Ed. Engl., 1995, 34, 2311.

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