FULL PAPER Croconic Acid and Alkali Metal Croconate Salts: Some New Insights into an Old Story Dario Braga,*[a] Lucia Maini,[a] and Fabrizia Grepioni*[b] Abstract: The solid-state structures of a are described and compared with the interplanar separations lie in the narrow series of alkali metal salts of the cro- Li ,K and NH4 salts. Single crystals of range 3.12 ± 3.42 ä and do not necessa- 2À p conate dianion (C5O5 ) and of croconic croconic acid were obtained by crystal- rily reflect the presence of -stacking acid (H2C5O5) have been determined. lisation of croconic acid in the presence interactions. It is argued that the small The alkali metal croconates were ob- of HCl. Crystal structure determinations interplanar separation is the result of a 2À tained by ring contraction of rhodizonic showed that the C5O5 ions tend to compromise between packing of flat acid (H2C6O6), upon treatment with organize themselves in columns. The croconate units and the spherical cations alkali metal hydroxides and recrystalli- together with the water molecules that sation from water. The novel species fill the coordination spheres of the alkali Keywords: alkali metals ¥ Na C O ¥2H O, Rb C O and Cs C O , metal atoms. 2 5 5 2 2 5 5 2 5 5 croconates ¥ crystal engineering ¥ as well as the mixed hydrogencroconate/ oxocarbons ¥ stacking interactions croconate salt K3(HC5O5)(C5O5)¥2H2O Introduction supramolecular aggregation of the building blocks rather than from their nature. Crystal engineering, the bottom-up construction of crystalline Indeed most of the successful crystal-engineering experi- solids with desired arrangements of the component molecules ments have been conducted on simple systems, like guanidi- and ions,[1] has fuelled new interest in some old issues of nium and sulfonate ions,[5] organometallic acids,[6] halometal- structural chemistry. Like supramolecular chemistry, defined late systems,[7] copper halides,[8] nanoporous systems[9, 10] and as chemistry beyond the molecule,[2] crystal engineering is coordination networks.[11, 12] We have followed a similar type concerned with the assembly of molecules[3] and ions in of approach in one of our lines of crystal-engineering aggregates of higher complexity, with collective properties research.[13] We used polyprotic organic and inorganic acids that depend upon the plethora of intermolecular interactions to exploit the robustness and reproducibility of hydrogen- that are responsible for crystal cohesion and stability.[4] The bonding interactions. A family of such acids that has proved to complex relationship between molecular size and shape (and be particularly well suited are the oxocarbon acids, which ionic charge in the case of ions) and the type and number of include rhodizonic (H2C6O6), croconic (H2C5O5), squaric [14] intermolecular interactions often favours the utilisation of (H2C4O4) and deltic (H2C3O3) acids. We have used the (relatively) simple ions and molecules as building blocks. monodeprotonation products of squaric acid to construct Simplicity will (hopefully) provide better insight into the interdigitated coordination complexes and hybrid organic ± factors that are truly important for crystal construction. organometallic materials for magnetic studies.[15] We were However provocative it may sound, chemical novelty is not a attracted by the structural characteristics of these simple and necessary prerequisite for a crystal-engineering building block elegant chemical systems. to be useful. Chemical and physical novelty arises from the Here we report the synthesis and structural characteriza- tion of a series of salts of the croconate dianion (C O 2À) and [a] Prof. D. Braga, Dr. L. Maini 5 5 Dipartimento di Chimica G. Ciamician the structure of croconic acid (H2C5O5). While the solid-state [16] [17] Universita¡ di Bologna structures of deltic and squaric acids were established Via Selmi 2, 40126 Bologna (Italy) long ago, that of croconic acid was only recently communi- Fax: (39)051-209-9456 cated.[18a] The croconate dianion belongs, together with the E-mail: [email protected] 2À 2À rhodizonate C6O6 , the squarate C4O4 and the deltate [b] Prof. F. Grepioni C O 2À ions, to the family of oxocarbon dianions. The Dipartimento di Chimica, Via Vienna 2 3 3 Universita¡ di Sassari, 07100 Sassari, Italy prototype of these dianions is the rhodizonate dianion 2À [18b] E-mail: [email protected] C6O6 , which has attracted the interest of many research- 1804 ¹ WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002 0947-6539/02/0808-1804 $ 20.00+.50/0 Chem. Eur. J. 2002, 8,No.8 1804±1812 ers in view of the structural analogy with benzene. Rhodiz- pressed by the strong surrounding coulombic field. This study onate salts have found many applications, for example as provides further examples of charge-compressed stacking of markers for lead, in the analysis of radium in fresh waters, and anions. also for their luminescence properties.[19] Yellow croconic acid was discovered, together with orange potassium croconate dihydrate (see below), by L. Gmelin in 1825.[20] Gmelin Results and Discussion himself derived the name ™croconic∫ from the Greek to c1oÂcon for saffron or egg yolk, because of the yellow and Table 1 summarises the compounds that are described in this orange colours of croconic acid and of many of its compounds. paper and provides references to those reported by others.[23] Gmelin wrote: ™Were it confirmed that it is a hydroxyacid, Relevant intra- and interionic structural parameters are then it should be given the name hydrocroconic acid and its compared. While the structure of K2C5O5 ¥2H2O was reported radical should be called crocon∫.[20] recently,[23b] the structures of the sodium, rubidium and cesium We have utilised oxocarbon anions for evaluating some salts are novel. fundamental aspects of hydrogen-bonding interactions be- tween ions.[21] Structural investigation on rubidium and Croconic acid: Yellow transparent single crystals of croconic cesium hydrogencroconates afforded some insight into the acid were obtained by crystallisation of croconic acid from an relationship between anion ± anion hydrogen-bonding inter- aqueous solution of HCl (1m; see Experimental Section). In actions and the presence of a coulombic field generated by the fact the first characterisation of the croconic acid molecule ions. It has been argued that interactions between ions are was not performed on the pure molecular crystal, but on a often the result of a compromise between the need to achieve serendipitous product obtained in the course of the reaction h5 maximum packing density and that of preserving weaker of the organometallic hydroxide [( -C5H5)2Co]OH with intermolecular (or interionic) interactions, which, however rhodizonic acid, followed by acidification of the solution with [18] h5 feeble with respect to the strength of the coulombic field HCl, which yielded the co-crystal [( -C5H5)2Co]Cl ¥ generated by small ions, are highly directional and contribute H2C5O5 . Rhodizonic acid is known to undergo ring contrac- to packing cohesion. This reasoning applies not only to tion upon treatment with bases, and this route yielded all the hydrogen bonding between ions but also to p stacking, as croconate compounds described here. shown previously for short interplanar separations observed The structure of croconic acid is shown in Figure 1 (top). in crystals of squarate and hydrogensquarate salts.[22] In these Importantly, all hydrogen atom positions could be obtained cases the weak noncovalent interactions are charge-com- from the diffraction data, and this allowed unambiguous Table 1. Relevant intra- and intermolecular parameters [ä] in the hydrogencroconate and croconate salts. À À Salt Interplanar Distance between Shift between C OCC Shortest O ¥¥¥ M Ocroconate ¥¥¥Owater < distance ring centroids the centroids interactions [ rvdW(O) rvdW(M)] [23a] [a] Li2C5O5 ¥2H2O 3.30 3.46 1.04 1.268(3) 1.451(2) 1.917(3) 1.990(4) 2.747(2) 1.245(2) 1.469(2) 1.984(3) 1.994(3)[a] 2.691(2) 1.235(2) 1.477(2) Na2C5O5 ¥2H2O 3.12 4.36 3.0 1.248(2) 1.461(2) 2.318(1) 2.586(1) 2.827(2) 1.249(2) 1.470(2) 2.468(1) 2.320(1)[a] 2.774(2) 1.242(2) 1.471(3) 2.507(1) 2.400(1)[a] [23b] K2C5O5 ¥2H2O 3.30 3.43 1.10 1.248(3) 1.474(2) 2.784(1) 2.870(1) 2.783(2) 1.241(2) 1.474(2) 2.870(1) 3.006(1) 2.877(2) 1.252(2) 1.465(3) 2.789(1) 2.649(1)[a] 2.864(1) 3.064(2)[a] [b] K3(HC5O5)(C5O5)¥2H2O 3.19 4.23 2.78 1.234(4) 1.477(5) 2.661(3) 2.910(3) 2.444(5) 1.233(4) 1.450(5) 2.719(3) 3.118(4) 2.972(6) 1.289(4) 1.435(5) 2.738(3) 3.352(5) 2.786(6) 1.243(4) 1.476(5) 2.767(4) 2.726(4)[a] 1.231(4) 1.473(5) 2.820(3) 2.809(4)[a] 2.847(4) Rb2C5O5 3.30 3.82 1.92 1.24(1) 1.458(7) 2.838(5) 2.892(5) 1.244(8) 1.463(9) 3.021(3) 3.135(5) 1.240(8) 1.47(1) 2.851(5) 2.958(6) 3.106(3) Cs2C5O5 3.42 4.02 2.11 1.231(9) 1.46(1) 3.029(5) 3.077(7) 1.226(8) 1.448(7) 3.126(4) 3.337(2) 1.261(9) 1.46(2) 3.048(6) 3.117(7) 3.235(6) [23c] [c] [c] (NH4)2C5O5 3.30 3.47 1.07 1.24(1) 1.46(1) 2.82(1) 2.83(1) 1.27(1) 1.45(1) 2.83(1)[c] 2.91(1)[c] 1.29(1) 1.45(2) [a] Owater ¥¥¥M .