Unstable radical anions dimerization

Various thermodynamic and kinetic problems concerning the chemistry and physics of radical anions and dianions were investigated quantitatively by electron photoejection. The approaches described allow the determination of the relative electron affinities of various aromatic hydrocarbons and the thermodynamics and kinetics of disproportionation of their radical anions into dianions. These approaches also allow the observation of unstable radical anions and their isomerization or dimerization. 

Chemical reduction of 2,4,6-tricyano-l,3,5-triazine, TCYT, results in the formation of an unstable radical anion that undergoes immediate dimerization at a ring carbon to form [Gi2Ni2], [TCYT]2, characterized by a long... 

Recently, an unsupported Ir dimer [IrCl2(CO)2]2 has been reported. This compound provides the opportunity to make linear mixed-valent Ir chains by electro-reduction. Linear polymeric ID chains incorporating (f Ru and Os have been reported. Electro-reductions of [M°(L)(CO)2Cl2] (L = 2,2 -bipyridine or 1,10-phenanthroline) produce linear chains [M(L)(CO)2] , obtained as adherent crystalline thin films on conductive supports. Polymerization occurs by an electrochemical propagation process (Scheme 60). The first step involves the reduction to an unstable radical anion that concurrently loses one Cl ligand and transforms to a coordinatively unsaturated species. The reactive 17e transient species rapidly forms dimer. Subsequent reduction of the dimer at the applied potential promotes further chain extension, leading to oligomers and polymers (Scheme 61). 

A further group of nonbenzenoid aromatics is the series of odd-membered cations and anions such as cycloprope-nium (14) and tropylium cations (15) as well as cyclopentadienyl (16) and cyclononatetracenyl anions (17). Regarding the arguments for the properties of Hiickel-like 4 + 2 jr-systems, all these molecules should be energetically stabilized. Obviously, this is not fulfilled in all cases. The tropylium cation (15) can be reduced in a one-electron step to the tropyl radical even at A = +0.06 V vs. SCE [85, 86]. The radical is unstable and rapidly dimerizes to bitropyl. The hep-taphenyl tropylium radical is stable on the voltammetric timescale, but decays... 

To utilize the strong reducing power of the 3(da po) excited states of the platinum and iridium dimers, the nonproductive back electron transfer reactions need to be inhibited. An effective way to accomplish this is to use acceptors that are thermally unstable after the initial electron transfer. Reduction of alkyl halides has been shown to lead to short-lived radical anions RXT, which rapidly decompose to give R- and X (k... 

An alternate mechanism involving reduced FAD as sensitizer [pathway B] indicates that an electron is directly donated to the pyrimidine dimer, forming FADH and dimer radical anion. Pyrimidine dimer radicals are unstable (157) and will spontaneously decompose to the monomer and monomer radical anion. The latter species then regenerates reduced FAD in a manner similar to that described for pathway (A). 

When X = CN, a stable anion-radical is obtained as evident from the ESR spectrum. When X = NO2, the ESR signal cannot be observed. This anion-radical is unstable and gives rise to the dimer (isolated Scheme 3.5). 

The polarogram of 1,4-dinitropyrazole is considerably more complex and has five waves (Table 3.36, Section 3.3). This compound is reduced more easily than all the investigated nitroazoles. The first wave corresponds to an irreversible one-electron transfer (Scheme 3.19). As with 1-nitropyrazole, at this stage an unstable anion radical is formed and then breaks up at bond N-N02. The N02 anion is reduced at potential -1.7 V. 4-Nitropyrazolyl radical further is dimerized with subsequent reduction [851],... 

During the electrolysis of a carboxylic acid 3, an electron is removed (at the anode) from the car-boxylate anion 1, leading to the corresponding carboxyl radical 5. This unstable species rapidly loses CO2, generating the C-centered radical 6. Coupling between two radicals affords the dimer 2 (Scheme 2) [13]. 

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