Ferrocene derivatives cation-radicals

Reversible one-electron oxidation of ferrocene (Fc) and its derivatives toward cation-radicals (the so-called ferrocenium cations) is a well-known reaction. The cation-radical center is localized at the iron atom. According to photoelectronic spectra and voltammetry data, such localization is... 

Bard and co-workers have reported on the attainment of equilibrium between the nanosized particles and an electrode in the presence of a redox mediator [25a]. The study refers to the production of a mediator (methyl viologen radical cation) that reduces water in the presence of colloidal gold and platinum metal catalyst. An electrochemical model based on the assumption that the kinetic properties are controlled by the half-cell reactions is proposed to understand the catalytic properties of the colloidal metals. The same authors have used 15 nm electrodes to detect single molecules using scanning electrochemical microscopy (SECM) [25b]. A Pt-Ir tip of nm size diameter is used along with a ferrocene derivative in a positive feedback mode of SECM. The response has been found to be stochastic and Ear-adaic currents of the order of pA are observed. 

Photo-induced additions of ferrocene to electron-deficient alkynes, giving 1-ferrocenylalkenes, photo-ethoxycarbonylation of ferrocene derivatives ( v-CCU-EtOH), and electrochemically-initiated additions of alkyl radicals (from RCOg ) to ferricenium cation have been reported. Electrochemical reductive cleavage of the ring-metal bonds in carbonyl-substituted ferrocenes has been achieved using polarographic methods. ... 

Both typical and exceptional examples of the polymerization of a vinyl monomer containing a transition-metal ion are provided by the radical polymerization of vinylferrocene31. Vinylferrocene and its derivatives are polymerized by a radical or a cationic initiator to form a polymer of high molecular weight. The high polymeriz-ability is based on the property that the ferrocene compounds are extraordinarily stable against chemical reactions. 

In addition to ferrocene, the oxidative redox couple that has received the most attention in supramolecular chemistry is tetrathiofulvalene (TTF), 35. This compound undergoes two reversible one-electron oxidations, first to a radical cation and then to a dication (Eq. 1.21). TTF first came to prominence in the 1970s when it was discovered that the charge transfer complex between it and 7,7,8,8-tetracyanoquinonedimethane (TCNQ) shows metallic conductivity. As a result, a large variety of different TTF derivatives have been prepared and characterized. This rich synthetic chemistry, coupled with the electroactivity, has intrigued supramolecular chemists for some time, with the result that the TTF unit has been incorporated into a wide variety of... 

Now let me come back to primary substitutions at the ferrocene nucleus. Together with Vil chevskaya, we phosphorylated ferrocene and its derivatives to triferrocenylphosphine oxides [263, 264). An unusual reaction, discovered in collaboration with Perevalova and Yur eva, was the direct cyanation of ferrocene with hydrocyanic acid in the presence of ferric chloride [265,272). Initially, cyanide attacks the iron atom of the ferricinium cation, then the cyanide group transfers to the ring while the iron is simultaneously reduced. The reaction was termed by us as the ricochet (from the metal to the nucleus) substitution it may be applied to many substituted ferrocenes and to the ruthenocenium cation [273), and it is now the simplest route to ferrocene carboxylic acids through their nitriles. Further, ferrocene was studied in acid-medium reactions with oxo compounds. With aldehydes [274), the reaction was complicated by the transformation of ferro-cenylalkyl carbinol formed Initially via the carbonium ion, followed by transformation to a radical which, in its turn, was coupled to 1,2-bis-(ferrocenylalkyl)ethane (27.5). The reaction with acetone led to 2,2-di-ferrocenylpropane (276). 

However, in contrast to benzene, ferrocene is sensitive to oxidation, and the ferrocenium cation, FeCpj, a paramagnetic 17-electron species, is readily formed in the presence of various oxidants. The ferrocenium cation is reluctant to undergo electrophilic substitution, and therefore reactions such as halogenation and nitration, which are important routes to substituted benzene derivatives, cannot be used for the synthesis of substituted ferrocenes. Only electrophilic substitution under nonoxidizing conditions (e.g., Friedel-Crafts acylation, Mannich reaction, borylation, lithiation or mercuration), and radical substitution are available as an entry into the chemistry of substituted ferrocenes. 

Calculations have been carried out on the spectroscopy and excited-state energies and geometries of polyenes,84 the 1,5-cyclo-octadiyne radical cation,85 the visual chromophore, ll-c/s-retinal,8 polyene carbaldehydes,87 aromatic hydrocarbon anions and cations,88 trimethylenemethane,89 thiamine derivatives,80 porphyrins,91 lumiflavin,92 chlorophyll,93 nucleic acids,94 and ferrocene.95... 

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