Croconate oxocarbons

As shown above, di-tert-butoxyethyne 1s the only acetylenic diether prepared so far whose stability allows its use as a synthetic Intermediate. It has been used 1n the synthesis of all the members of the series of monocyclic oxocarbons (deltic, squaric, croconic, and rhodizonic acids), as well as in the synthesis of semisquaric acid, the parent compound of the natural mycotoxins, noniliforndna... 

The rhodizonate dianion C60g (Fig. 20.4.19) is a member of a series of planar monocyclic oxocarbon dianions C 0 (n = 3, deltate n = 4, squarate n = 5, croconate n = 6 rhodizonate) which have been recognized as nonbenzenoid aromatic compounds. However, this six-membered ring species... 

Of the known cyclic oxocarbon acids, the systems based on squaric (68) and croconic (69) acids have been most widely studied. The loss of two protons from these acids gives rise to aromatic dianions as shown in equations (18) and (19), and these can coordinate to metal anions in a variety of ways. Unidentate coordination (70,77) is known for both systems but is not common. Simple bidentate chelate coordination (78) is also relatively uncommon but is observed in a number of croconate complexes. The squarate anion adopts this mode only with larger cations, such as the group 2 and lanthanide metals, and then only in association with additional bridging interactions. Bridging coordination modes dominate the chemistry of these anions, some of which are shown here (71-76), (79-81). The various modes of coordination can usually be distinguished by IR spectroscopy, and the use of NMR spectroscopy has also been investigated. 

The effect of surface structure, the role of the various crystal faces polyols on gold, sorbitol at polycrystalline and single crystal platinum electrodes, 2,3 butanediol stereoisomers at platinum single crystal electrodes, studies on the behavior of oxocarbons (squaric and croconic acids) on single crystal platinum surfaces. 

Gas phase SCF energies of the U(lV)-bound oxocarbon series [U(COT) (Cp)l2(CO) (n = 3-6) have been calculated, and are illustrated in Fig. 7. Energies of rf-.rf -hound, croconate and rhodizonate complexes were found to be... 

The deconstruction of the potassium croconate crystal allows us to recognize the analogies with another member of the family of oxocarbon anions, rubidium rodizhonate Rb2C606. The crystals possess some remarkable... 

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). 

Fig. 16.35 Cyclic oxocarbon unions (a) squarate (b) croconate (c) rhodizonaie.

Oxocarbons are organic compounds consisting only of carbon and oxygen in their neutral state, they exist as diacids, and their anions possess the general formula where n>2 and generally =3-6. Oxocarbons have been the focus of research since the nineteenth century, when the croconate (3, w = 5) and rhodi-zonate (4, n = 6) anions were first synthesized [1]. The squarate anion (2, w = 4) was later synthesized by Cohen et al. in 1959, and the oxocarbon deltate anion (1, K = 3) was synthesized by Eggerding and West in 1976 (Figure 5.1) [2]. 

Figure 5.2 Electronic spectra of oxocarbons (a) squarate 2, (b) croconate 3, and (c) rhodi-zonate 4 dianions in aqueous solutions.

Patton and West studied the electrochemistry of these species. The radical anions of squarate 2, croconate 3, and rhodizonate 4 were characterized in dichloromethane using the electron paramagnetic resonance (EPR) technique [17]. Likewise, Carr, Fabre, and collaborators obtained the UV/visible and EPR spectra of these radical dianions, produced electrochemically in dimethylfor-mamide [18]. The oxidation potential of the oxocarbonic acids was determined in perchloric acid solution using platinum electrodes. The oxidative process was proposed to proceed in two stages, beginning with the transfer of charge from substrates at the electrode. Subsequently, the oxidation product is desorbed from the electrode and hydrated [19]. 

Recently, Tsoureas and coworkers [27] described the use of a mixed-sandwich U(III)-complex in the reductive coupling of CO. In this work, the authors demonstrate the effect of the steric environment around the uranium center in the reduction process by the complex in formation of oxocarbons, in particular squarate 2 and croconate 3 species. 

The most studied pseudo-oxocarbons derived from the croconate dianion 3 are croconate violet [3,5bis(dicyanomethylene)cyclepentane-l,2,4-trionate] 17 and croconate blue [2,4,5-tris(dicyanomethylene)cyclepentane-l,3-dionate] 18 [16b, 17-19]. They can be obtained from the reaction between croconate 3 and malononitrile (H2C(CN)2), as can be seen in Scheme 5.3. These derivatives are of interest because of their reversible electrochemical character [20, 21], their strong absorption maxima in the visible region, which determine their intense colors (molar absorptivity in the range of 10 lmol cm ), their delocalized Jt-system,... 

Figure 5.5 The most common pseudo-oxocarbons derived from croconate 3 1,2-ditihocroconate 14, l,3-bis(cyanoimine) croconate 15, l,2-bis(cyanoimine)croconate 16, 1,2,3-tris(dicyanomethylene) croconate 17, and 1,2-bis(dicyanomethyiene)croconate 18.

Pseudo-oxocarbons coordination chemistry has hardly been described in the chemical literature. Croconate violet 17 exhibits four different coordination modes as can be inferred from Table 5.5. 

Some complexes with croconate violet 17 display intramolecular interactions in which the metal transmits electronic properties for electrocatalytic or electrochromic applications [44b]. Such intramolecular interactions can be understood as the most important driving forces existing in the solid state for the coordination compound, where the electronic properties are localized over the oxocarbon moieties. Croconate violet 17, croconate blue 18, and lithium croconate are highly soluble in water and also in many nonaqueous solvents, thus restricting their practical applications [59]. However, this solubility can be circumvented in the case of croconate violet, for example, by incorporating it in a protonated film of poly(4-vinylpyridine), which, after adequate treatment, exhibits interesting electrocatalytic properties [60]. 

Numerous studies have been conducted on the synthesis of compounds substituted by dicyanomethylene groups, as shown in Figure 5.8 [1, 48b, 82]. Malononitrile is often used in substitution reactions of oxocarbon systems, especially in the croconate and squarate series [83], as shown in Schemes 5.2 and 5.5. 

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