Pseudo-oxocarbons

For these compounds it was verified that the greatest difference in CC bonds (ACC) of the pseudo-oxocarbon ring lies in the range of 0.039-0.053 A, except for Cs salts which showed the largest ACC (0.107 A). These values indicate that 

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. 

The most common coordination mode is by way of neighboring oxygen atoms (motif XXVI). These function as a chelate and have been observed with the transition metal salts of Co, Cu, and Zn as well as those of La, Nd, Gd,  

---

I 5 Oxocarbons, Pseudo-oxocarbons, and Squaraines Table 5.4 continued)... 

Oxocarbon derivatives either totally or partially substituted by phosphorus and selenium atoms or by nitrogenous groups are examples of the so-called pseudo-oxocarbons [4,49]. Sulfur derivatives are also among these compounds, with the nature of the substituent group resulting in substantial electron delocalization, which is an interesting feature in the context of optical materials with nonlinear properties [20]. The most studied pseudo-oxocarbons and their respective oxocarbons are depicted in Figures 5.3-5.6. 

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 53 The most common pseudo-oxocarbons derived from deltate 1 tris(cyanoimino)deltate 7, tris(dicyanomethylene)deltate 8, and tritiodeltate 9.

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.

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.

Figure 5.6 The most common pseudo-oxocarbons derived from rhodizonate 4 tetra(amine)-p-benzoquinone 19, hexathiorhodizonate 20, and tetracyanoquinodimethane 21.

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]. 

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. 

Oxocarbons, pseudo-oxocarbons, and squaraines are a very versatile class of compounds, being useful in a wide range of areas, from model compounds to medicinal chemistry. Their role in coordination of metal ions appears to be their most important property. For the coming years, oxocarbons and their derivatives will be very important in crystal engineering. For this application, their impressive ability to provide several different coordination modes is particularly important. 

---