Croconate violet

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,... 

Table 5.5 Schematic representation of the coordination modes exhibited by croconate violet 17 dianion.

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]. 

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