Rhodizonate dianions

In the final section of this chapter, we discuss the vibrational spectra of benzene, its isostructural species CeOg- (rhodizonate dianion), and dibenzene metal complexes. 

Benzene and the rhodizonate dianion (C6C>62-, structure shown above also see Section 20.4.4) are isostructural species. Hence all the symmetry arguments and results given in the previous section for benzene are applicable here. The results for C6C>62- are summarized in Table 7.3.5. 

Table 7.3.5. The normal modes of the rhodizonate dianion and observed frequencies...

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... 

Bond lengths (pm) of (a) D6tl (note that in this case the dianion is located at a site of symmetry) and (b) C2v valence tautomers of the rhodizonate dianion. 

OH- is not the electron donor, but instead 7-benzoquinone itself acts as the electron donor in the presence of OH. The addition of OH" to p-benzoquinone initiates an electron transfer from the OH adduct of p-benzoquinone to p-benzoquinone leading to the noble disproportionation. This yields ten equivalents of semiquinone radical anion and rhodizonate dianion [176],... 

Braga. D. Cojazri. 6. Maini, L. Grepioni. F. Reversible solid-state interconversion of rhodironic acid H2C6O6 into H6C60g and the solid-state structure of the rhodizonate dianion C606 (aromatic or non-aromatic ). New J. Chern. 2001. 25. 1221-1223. 

In the host lattice of 24, the rhodizonate dianion resides at an inversion center, being directly linked to a pair of A -(3-hydroxyphenyl)urea molecules through pairs of N-H- -0 [A, N2 = R 9)] and phenyl C-H- -0 [B, N2 = f 2(6)] hydrogen bonds... 

Figure 8.49 Projection along the b axis showing extensive hydrogen bonding interactions around the centrosymmetric rhodizonate dianion in the host lattice of [(n-C4Hg)4N ]2C606 2(m-OHC6H4NHCONH2)-2H2O (24). Symmetry transformation a a - X, i - y, 1 - z)...

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. 

This study provides the first reasonably precise molecular dimensions of the rhodi-zonate dianion, which lends support to the aromaticity of this nonbenzenoid cyclic oxocarbon. Notably, the measured C-C bond lengths [1.421(5) — 1.458(5) A] of the rhodizonate in 21, which exhibits approximate D(,h molecular symmetry, are significantly shorter than the corresponding values (1.488 and 1.501 A) in Rb2C60e [46] and the calculated values (1.500 and 1.501 A) for the C2 structure of this dianion [47]. Compound 21 provides yet another example of the use of urea and its derivatives for stabilizing elusive molecular anions such as allophanate [5h] and dihydrogen borate [5c] in a hydrogen-bonded host lattice. 

Figure 5.1 Chemical representation of deltate 1, squarate 2, croconate 3, and rhodizonate 4 dianions.

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]. 

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