Croconate dianion

Hydrogen-bonding environment of the croconate dianion in the crystal structure of[(C2H5)4N+]2C502-.3(CH3NH)2CO. 

The MCD spectrum of the croconate dianion CjO, is shown in Figure 3.10. The molecule possesses a fivefold symmetry axis and belongs to the point... 

Figure 3.10. MCD spectrum (top) and absorption spectrum (bottom) of the croconate dianion C,0, (by permission from West et al., 1981).

Similar arguments apply to other highly symmetrical aromatic compounds with only two electrons or two Jt holes. Thus, the positive A term of the croconate dianion C,0, (7) may be rationalized by realizing that it can be formally written as a five times deprotonated pentahydroxy derivative of the trication CjH, . (Cf. Example 3.6.)... 

Deprotonation of rhodizonic acid (132) produces a dianion (133) which possesses aromatic character. Photolysis of this dianion in aqueous solution containing electron acceptors is found to yield croconate dianion (134), and the quantum yield of the reaction has been measured for various concentrations of the acceptors. The mechanism of the reaction is uncertain but may involve ring contraction following electron transfer and addition of water. 

Another example is afforded by potassium croconate K2C5O5, which was isolated and characterized as K2C5O5. 2H2O by Dunitz and subsequently by us. The coordination around the potassium cation is depicted in Fig. 9a. The croconate dianions form columns, extending parallel to the c-axis. completely surrounded by cations and water molecules, with these latter acting as "pinchers" on the dianions (see Fig. 9b.c). The short interplanar separation (3.30 A) is thus a result of the external interactions that "compress" the anions together. 

Fig. 9 Crystal deconstruction in K2C5O5.2H2O (a) the coordination of two water molecules and four croconate dianions around the K cation in crystalline (b) the dianions are stacked in columns surrounded by the cations and the water molecules (b) the latter act as pinchers" along the columns (c). (From Refs. [37,38].)...

Figure 8.47 Hydrogen bonding scheme of 23, showing a wide croconate-phenylurea ribbon running parallel to the b axis. The disordered croconate dianion lies at an inversion center for clarity, one orientation is shown on the left and the other on the right Symmetry transformations a (I — x, —y, 1 — z), b (1 — x, 1 — y, 1 — z) and c (x, 1 + y, z)...

Nonbenzenoid aromatic Dsf, valence tautomer of croconate dianion. In the crystal structure of 26, two of the three independent urea molecules constitute a hydrogen-bonded dimer, labeled motif A (Figure 8.54). Adjacent pairs of urea dimers lying parallel to the c-axial direction are twisted by 60° and bridged by pairs of water molecules derived from Olw and 02w via pairs of strong -O and O-H O hydrogen bonds [58a]... 

Table 5.4 Coordination modes of croconate dianion 3 toward transition metals and lanthanides.

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,... 

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