Dianion/dication

A very thorough study of the solvent acid acidity dependence of the absorption and fluorescence of the plant estrogen coumestrol (22), has been made by Wolfbeis and Schaffner. In aqueous solution, five different species (dianion dication) can be involved. Other related compounds have also been examined. The yellow-green luminescence from quercetin and 3-hydroxyflavone at room temperature has... 

Bamberger, 4 Benzoannulenes, 331 Benzoannulenes, dehydro, 332 Benzobiphenylenes, 230 Benzocyclobutadienes, 216 Benzocyclobutadienes, dianions, dications, 220 Benzocyclononatetraenide salts, 265... 

Mullen, K. Meul, T. Schade, R Schmickler, H. Vogel, E.J. Am. Chem. Soc., 1987,109, 4992. This paper also reports a number of other bridged paratropic 12-, 16-, and 20-electron dianions and dications. See also Hafner, K. Thiele, G.F. Tetrahedron Lett., 1984, 25, 1445. 

Experimental evidence for the six electron systems has been described in Sect. 2.1.4. Skancke reproduced the relative stabihty of the cross conjugated systems relative to the linear isomers by calculating the trimethylenemethane and buta-l,4-diyl dianions [27] and their dilithio salts [28]. For the four electron systems butadiene is more stable than trimethylenemethane. Experimental examination of the relative stabihties of two electron systems using the trimethylenemethane and buta-14-diyl dications needs to overcome the intrinsic instabihties of dications dissatisfying the octet rule. 

The CVA method shows that electrochemical reduction of 3,3 -bis(2-R-5,5-dimethyl -4-oxopyrrolidinylidene)-1,1 -dioxides (237) (Fig. 2.20), which per se are dinitrones conjugated with a C=C double bond, is an EE process that produces stable radical anions and dianions. Oxidation is an EEC- or EE-process that gives stable RC and dications (431, 432). 

Our success in super-stabilization of cation 6 led us to the preparation of a higher homologue, that is, cyclooctatetraene (COT), fully annelated with BCO units 9 (9). As compared with a large number of studies on its radical anion or dianions, the studies on the cationic species of COT have been quite limited. There have been only one study by Olah and Paquette on the substituted COT dication (70), which is a typical 6n Hiickel aromatic system, and few sporadic studies on radical cations, which involve indirect spectral observations, such as electronic spectra in Freon matrix at low temperature (77,72) and constant-flow ESR study (13). 

Fluorenylidene dications, such as the dications of p- and m-substituted diphenylmethylidenefluorenes, show appreciable antiaromaticity. Evidence of antiaromaticity is demonstrated through H NMR shifts, nucleus independent chemical shifts (NICS), magnetic susceptibility exaltation, A, and (anti)aromatic (de)stabilization energies, ASE. Extension of the research to indenylidenefluorene dications shows that, contrary to expectation, the indenyl cation in these dications is less antiaromatic than the fluorenyl cation. The magnitude of the antiaromaticity is evaluated through comparison to the aromaticity of related dianions and reveals that the fluorenylidene dications are more antiaromatic than the fluorenylidene dianions are aromatic. 

The previous sections have demonstrated the antiaromaticity of a variety of indenyl and fluorenyl cations and dications but there has been no attempt to evaluate the degree of antiaromaticity. That is, are these species very antiaromatic or not particularly antiaromatic at all We need a context for the numbers and the corresponding aromatic dianions provides that context. 

Distorted n MOs can also be found in 4c2e bonding. Expanding 2A by a CH+ unit leads to the non-planar 7r,er-distorted aromatic C2B2H4 J, isoelectronic with the (CH)42+ dication [11] G and the B4H42 dianion Q, which all can be considered as distorted tetrahedra as well as folded rings. All have ten SE and classify as closo-clusters as shown for G and J in Figure 3.2-1. 

Accordingly, as illustrated in Figure 36 for [Pd o-C6H4(NH)2 2L these complexes display the step-by-step reversible sequence from the dication to the dianion.63... 

Chiral 2-imidazoline dianions undergo one-electron oxidation in the presence of TEMPO (2,2,6,6-tetramethyl-l-piperidinyloxy) to form a radical anion that is either trapped stereoselectively by TEMPO or undergoes dimerization. Oxidation of bis-diazene oxides leads to novel (9-stabilized 4N/3e radical cations and 4N/2e dications. These were detected by ESR spectroscopy and cyclic voltammetry. B3LYR/6-31G calculations confirmed the nature of the 4N/3e and 4N/2e systems. ... 

Cation-radicals containing three electrons in the field of four atoms or anion-radicals with five electrons retained by four atoms represent a special group of multicentered ion-radicals. Thus, nonclassical, cyclically delocalized 3e/4C cation-radicals and 2e/4C (dication)-radicals of substituted cyclobutadienes are known (Allen and Tidwell 2001). The 3e/4N cation-radical and the 5e/4N anion-radical have also been discovered (Exner et al. 1998, 1999, 2000). The reactions in Scheme 3.31 illustrate structures of the 5e/4N anion-radical as well as the corresponding dianion and acety-lated products of the latter. 

In a similar manner, p-bis(9-anthryl) phenylene gives a mono(anion-radical) or a mono(cation-radical) under reductive or oxidative conditions with spin delocalization around the whole molecular framework. In the case of m-bis(9-anthryl) phenylene, reduction or oxidation leads to the formation of dianion or dication-diradicals. Based on ESR experiments at cryogenic temperatures (6.5-85 K), these species contain two separated ion-radical moieties. They have parallel aligmnent of their spins (Tukada 1994). The work gives clear experimental evidence for the so-called ferromagnetic interaction between these ion-radical substituents. In some cases, release of the electron depends on temperature. See, for example, the anion-radical shown in Scheme 3.63. 

Fig. 2 Schematic representation of the doping of poly(p-phenylene ethynylene) (PPE) under formation of polarons (radical cations, radical anions) and bipolarons (dications, dianions). Note that multiple carriers can coexist on each macromolecule...

One could expect diazadiboretidines to be converted into Huckel aromatic systems either by adding or by subtracting one pair of n-electrons. The addition of two electrons to diazadiboretidines of the type (RBNalkali metals. The dianions [(RBNiBu)2] are stable in solution and can be reconverted into the diazadiboretidines by oxidants. Because they contain six n-electrons, aromatic character may be attributed to the dianions (19). Cyclodimers of the type (R BNR)2 are also readily oxidized, but the adoption of an aromatic dication [(R BNR)2] as a product would be mere speculation at present. 

This section is devoted to heterocyclic molecules isoelectronic with dications and dianions of the molecules that are central to the aromaticity concept, i.e., cyclobutadiene and benzene (Scheme 5). 1,3-Diboretene... 

THF tetrahydrofuran, NaNap sodium naphthalenide, Na2St disodium stilbene dianion, BP benzoyl peroxide, DME 1,2-dimethoxyethane, TMBD tetramethylbenzidine dication diperchlorate, WBP Wurster s blue perchlorate, LP lauroyl peroxide, FLSPEC fluorescence spectrum obtained, FPSPEC fluorescence and phosphorescence spectrum obtained, FXSPEC fluorescence and eximer spectra obtained, DPAC12 9,10-dichlorO 9,10-dihydro-9,10-diphenylanthracene. 

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