Squarate oxocarbons

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... 

The most dramatic synthetic achievement with cyclobutenediones is the synthesis of deltic acid derivatives. As illustrated below, photolysis43 of diethyl squarate (16 d) yielded, among other products, diethyl debate (21). The bis-trimethylsilyl ester (16 e) of squaric acid similarly furnished a deltate ester which was successfully hydrolysed to deltic acid itself (22), the lowest member of the series of oxocarbon acids48). 

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. 

Uranium(Vl) strongly bonds squarate (1), second in strength only to Fe(III) among the metal cations studied, and likewise rhodizonate (3), second only to Cu(II) , again among the metals of interest, which did not include Fe(III). The order of binding strength of the various metals to these oxocarbon anions remains unexplained. 

Fig. 16.35 Cyclic oxocarbon onions (a) squarate (b) croconaie (c) rhodizonaie.

This is the first synthesis of an oxocarbon from a CO2 carbon source and its synthesis may be considered as the product of successive 2e" reductions of CO2. The first reduction gives carbonate plus CO, the second then reduces this liberated CO to the squarate dianion (Eq. 2). However, it must be noted that... 

In continuing and most elegant studies, Mak and coworkers have exploited crystal engineering principles to capture and thereby stabilize hitherto rare or even uncharacterized species within a crystalline manifold.For example, monocyclic oxocarbon dianions C 0 for n = 3, 4, 5, and 6 have been characterized in this manner. As an illustrative example, the squarate dianion, 404 (Figure 26a), has been trapped in a crystal structure with formulation PiUN]2[C404 ]- 4(NH2)2C=S 2H20. The anions are located in a three-dimensional host lattice whereby zigzag... 

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. 

Figure 5.4 The most common pseudo-oxocarbons derived from squarate 2 tetrathiosquarate 10, 1,2-dithiosquarate 11, tetra(cyanoimine)squarate 12, and carbosquarate 13.

In addition to the previously mentioned pseudo-oxocarbons, derivatives of the squarate ion 2 (or l,2-dihydroxycyclebutene-3,4 dione 6 in the acid form) have been prepared with carbon chains, nitrogen, sulfur, and selenium as substituent species [61]. The study of thio-derivatives of pseudo-oxocarbons demonstrates the interest to understand the characteristics and chemical behavior of these derivatives, which could be useful for the preparation of reduced dimension materials with metallic or semiconductive properties [4]. 

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. 

Squarate compounds with transition metal ions are rarely described in the chemical literature. For example, the crystal structure of cis-bis(dicyanomethylene) squarate 27 in a complex with Cu(I) has been reported [48b]. Galibert and collaborators synthesized a complex of Cu(II) with fraws-bis(dicyanomethylene) squarate 26 [86]. The pseudo-oxocarbon ring in this complex was found to be planar. However, a significant deviation of the cyano groups from the best plane formed by the ring contributes to the diminished planarity of the complex, a phenomenon that is also reflected by the loss of Jt-delocalization in the ring. 

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