Radical anions dications

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). 

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. ... 

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...

Almost all reactions of alkylidenecycloproparenes lead to opening of the cyclopropane ring. A notable exception to this is the reversible electrochemical reduction of237 and 240 which leads to the stablfe radical anions 396 and 397, with half-wave potentials of -2.32 and -1.93 V, respectively, and their oxidation to the quasi-stable radical cations 398 and 399 ( i/2(ox) = +0.68 and +0.81). The cations may be further oxidized to the corresponding very short-lived dications. In contrast, the photoelectron spectra of 237 and 240 reveal practically identical first-oxidation potentials of both compounds, which indicates that the difference in half-wave potentials for oxidation (in condensed phase) of 237 and 240 does not exist in the gas phase. This has been attributed to structure-specific solvation energies in the radical cations 398 and 399. °... 

The structure and energy of a series of ions generated from penta-cyclo[3.3.1.13,7.01 3.05 7]decane (7) has been explored by using HF, MP2 and DFT methods to estimate enthalpy changes of isodesmic disproportionation reactions and by considering the reorganization of frontier orbitals as a consequence of addition or removal of electrons from the neutral molecule.8 The dication (72+), which is considered to be Three-dimensionally homoaromatic , is stable relative to a localized structure with similar features but is highly unstable compared to the radical cation (7+i)- hi contrast, the dianion (72 ) is unstable relative to the radical anion (T) and shows no evidence of electron delocalization. 

Since electrode measurements involve low substrate concentrations, reactive impurities have to be held to a very low level. The physical data and purification methods for several organic solvents used in electrode measurements have been summarized (Mann, 1969). But even when careful procedures for solvent and electrolyte purification are employed, residual impurities can have profound effects upon the electrode response. For example, the voltam-metric observation of dications (Hammerich and Parker, 1973, 1976) and dianions (Jensen and Parker, 1974, 1975a) of aromatic hydrocarbons has only been achieved during the last ten years. The stability of radical anions (Peover, 1967) and radical cations (Peover and White, 1967 Phelps et al., 1967 Marcoux et al., 1967) of aromatic compounds was demonstrated by cyclic voltammetry much earlier but the corresponding doubly charged ions were believed to be inherently unstable because of facile reactions with the solvents and supporting electrolytes. However, the effective removal of impurities from the electrolyte solutions extended the life-times of the dianions and dications so that reversible cyclic voltammograms could be observed at ambient temperatures even at very low sweep rates. 

A dialkylated derivative of the end-capped sexythienyl 3M allows for a comparison of the oxidized with the reduced states. The reduction leads to radical anions and dianions with similar optical absorption features than observed for the dications, but they are shifted slightly bathochromically (0.1 eV) [153]. In contrast to the cation radicals the anions fail to show any tendency toward dimerization (see Sect. 5). 

Ferrocenyl derivatives of 73, namely (Co3(CO)9 L (/i3-CFc)] (n = 0-3, L = phosphine or phosphite) undergo reversible reduction at the tricobalt center to detectable radical anions (n = 0), and oxidation at the iron center to give isolable cations (182). For the trisubstituted derivatives [ = 3, L = P(OMe)3 or P(OPh)3], the monocations appear to be mixed-valence species in which the two redox sites are weakly interacting low-energy absorption bands, assigned to intervalence transfer transitions, have been observed. The dications (Co3(CO)6 P(OR)3 3(/i3-CFc)] (R = Me or Ph) are also isolable, oxidation having occurred at both possible redox sites (183). 

For radical cations this situation is typically observed when deprotonation of the dimer dication is slow and for radical anions under conditions that are free from electrophiles, for example, acids, that otherwise would react with the dimer dianion. Most often, this type of process has been observed for radical anions derived from aromatic hydrocarbons carrying a substituent that is strongly electron withdrawing, most notably and well documented for 9-substituted anthracenes [112,113] (see also Chapter 21). Examples from the radical cation chemistry include the dimerization of the 1,5-dithiacyclooctane radical cations [114] and of the radical cations derived from a number of conjugated polyenes [115,116]. 

In most cases, the radical cations were unstable and decayed within milliseconds. The rate of decay was greatly influenced by the nature of the substituents on the porphyrin ring, as outlined earlier for the various radical anions. Thus, the tetranegative Zn TSPP gave a relatively long-lived radical cation, with half-life —7 s at pH 7. In contrast, the tetrapositive Zrf TMPyP complexes gave very short-lived radical cations. They decay via disproportionation to form a dication. 

After the porphyrin has been excited, it can lose an electron to form a radical cation, or it can gain an electron to form a radical anion. Each of these species is highly reactive in its own right and they rapidly and irreversible disproportionate in water to give, respectively, dications and... 

The cation and anion of neutral ethenedione are well-known. The radical cation, OCCO, was prepared by the reaction of CO with CO, isolated in a neon matrix and studied by ESR supplemented by ab initio calculations [38], and it has also been generated by mass spectrometry [39, 40]. The dication is known too [41, 42]. The radical anion was been reported and subjected to extensive theoretical analysis... 

Fig. 2. Molecular states and relevant measurement data Total energy scale for electronic ground (F) and excited (XI) states of a neutral molecule M and its radical cations M" or dications generated by ionization or oxidation or its radical anions M " and dianions resulting from electron insertion. In addition, the measurement methods used by the Frankfurt Group are indicated, especially UV, PES, ESR, and ENDOR spectroscopy as well as polarography (POL) or cyclic voltammetry (CV).

The radical cations and radical anions may homodimerize to give dications and dianions or heterodimerize to give diradicals. Homodimerization presumably occurs in the copolymerization of vinyl ethers, CH2=CHOR, with vinylidene cyanide, CH2=C(CN)2. On mixing these two monomers, a mixture of poly(vinyl ether) and poly(vinylidene cyanide) is produced, which can be explained in terms of the formation of dications and dianions, which then polymerize the respective monomers ... 

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