Croconate ion

Interestingly, the hydrogen bonding pattern in the crystal structure of 23 (Figure 8.47) is very similar to that in 21 (Figure 8.44), and the cyclic pentameric structural unit with an oxocarbon core is basically retained, so that the pair of inclusion compounds exhibits an isostructurality relationship [14]. However, the croconate ion is disordered about an inversion center, and it adopts two equally populated orientations. Consequently, there are only two independent phenylurea molecules in the asymmetric unit of 23, and the length of the c axis is about half of that in 21 (Figure 8.48). 

The structural simplicity of species 1-4 is further confirmed in the solid state through single crystal X-ray analysis [15]. Based on these studies, it was concluded that the squarate ion 2 exhibits molecular symmetry, the croconate ion 3 exhibits molecular symmetry and the rhodizonate ion 4 exhibits Z)g, molecular symmetry. Additional spectroscopic studies concluded that deltate... 

Table 5.2 Theoretically expected and observed bands for croconate ion 3 by both Raman and infrared vibrational techniques. ...

Spectroscopic technique Croconate ion (theoretical) Croconate ion (experimental)... 

The single-crystal X-ray analysis of potassium squarate monohydrate, determined in 1964, confirmed its symmetry [15cj. The D51, symmetry of the croconate ion 3 was confirmed in the same way at about the same time. 

The four-, five-, and six-membered analogs (178,180, and 182) were also obtained from the diprotonation of squaric acid (3,4-dihydroxy-3-cyclobutene-l,2-dione, 177), tri-O-protonation of croconic acid (4,5-dihydroxy-4-cyclopentene-l,2,3-trione, 179), and tetra-O-protonated rhodizonic acid (5,6-dihydroxy-5-cyclohexene-l,2,3,4-tetraone, 181), respectively. These ions were prepared in either Magic Acid (1 1 FSOsH-SbFs) or fluorosulfuric acid at low temperature and characterized by NMR. Ab initio/IGLO calculations showed that di-O-protonated squaric acid (178) is planar and aromatic, whereas the polyprotonated croconic and rhodizonic acids (180 and 182) have more carboxonium ion character, and no indication was obtained for any significant contributing homoaromatic structures. 

A combined theoretical and experimental studyshowed that solid complexes of Li, Na, K and Rb with the squarate ion (1) are neither isostructural nor similarly hydrated. As the size of the cation increases, the C—C bond lengths tend to equalize, which was interpreted by the original authors as an enhancement of aromatic delocalization. Croconates (2) appear more stable than rhodizonates (3). ... 

The electronic spectra of the potassium salts of squarate 2, croconate 3, and rhodizonate (4) ions in aqueous solution exhibit bands with high molar absorptivity, in the order of 10 1 mol cm , and a similar profile for all the spectra, with unfolded bands (Figure 5.2). 

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