Electron-reservoir complex

The I9e electron-reservoir complexes Fe Cp(arene) can give an electron to a large number of substrates and several such cases have been used for activation. After ET, the [FenCp(arene)]+ cation left has 18 valence electrons and thus cannot react in a radical-type way in the cage as was the case for 20e Fe°(arene)2 species. Thus the 19e Fe Cp(arene) complexes react with the organic halide RX to give the coupled product and the [FeCp(arene)]+ cation. Only half of the starting complex is used e.g., the theoretical yield is limited to 50% [48] (Scheme VI) contrary to the reaction with Fe°(arene)2 above. 

The special salt effect is a constant feature of the activation of substrates in cages subsequent to ET from electron-reservoir complexes. In the present case, the salt effect inhibits the C-H activation process [59], but in other cases, the result of the special effect can be favorable. For instance, when the reduction of a substrate is expected, one wishes to avoid the cage reaction with the sandwich. An example is the reduction of alkynes and of aldehydes or ketones [60], These reductions follow a pathway which is comparable to the one observed in the reaction with 02. In the absence of Na + PFg, coupling of the substrate with the sandwich is observed. Thus one equiv. Na+PFg is used to avoid this cage coupling and, in the presence of ethanol as a proton donor, hydrogenation is obtained (Scheme VII). 

Astruc has developed the concept of transition metal sandwiches acting as electron reservoir complexes. The characteristic of an electron reservoir is that the reduced form is easily generated and does not decompose to increase stability, the radical center can be sterically protected in the heart of a bulky molecular framework. The [FeCp(arene)] series of complexes, e.g. 7, are prime examples, for which variation of the arene structure modulates the redox potential. 

A very interesting recent study describes the intramolecular disproportionation of homodinuclear and heterodinuclear fulvalene complexes in the presence of PMe3.88 Equation (14) shows one of the six reactions reported. In this case, initiation of the radical chain process was accomplished with a catalytic amount of the 19-electron reservoir complex [CpFe(C6Me6)], which reduces 13 to break the Ru—W bond and generate a 17-electron radical center (presumably at Ru). Addition of PMe3 to the Ru is followed by electron transfer to the reactant (13) to afford the zwitterionic product 14 and regenerate the radical intermediate. 

D. Astruc, Organoiron Electron-Reservoir Complexes, Acc. Chem. Res. 19, 377-383 (1986). 

Volume 2 is dedicated to a detailed description of the most important classes of electron transfer reactions involving organic molecules (Part 2.1) and organometallic and inorganic compounds (Part 2.2). In several cases the reactions described are important not only from the viewpoint of fundamental research on reaction mechanisms, but also for their catalytic and synthetic applications. The emerging fields of electron transfer reactions of fullerenes, electron-reservoir complexes, and biomi-metic electron transfer chemistry of porphyrins are discussed in depth. 

Electron-transfer Reactions of Electron-reservoir Complexes and other Monoelectronic Redox Reagents in Transition-metal Chemistry... 

Electron-transfer Reactions of Electron-reservoir Complexes... 

The most frequently used electron-reservoir complex for stoichiometric singleelectron transfer reactions is (Fe Cp( / -C6Me6)], because of its stability and ease of preparation, and since it has one of the most negative redox potentials in the series. It can reduce most inorganic and organometallic cations [2]. For instance, it is very useful to synthesize neutral 19-electron complexes (C in the equation below) such as other (Fe Cp( / -arene)] complexes and (Fe ( / -C6Me6H)( / -C6Me6)] from the 18-electron cationic precursors C[PF6 ... 

The efficiency of the simple Na+ salts to inhibit the reactivity of 02 is very spectacular, and it matches the reactivity of superoxide dismutase enzymes in biological systems [173]. It is thus a unique property of the electron-reservoir complexes which lets us investigate the electron-transfer to O2 in various media. The follow-up reactions of superoxide radical anion are reminiscent of its damage for cells in the aging processes which is well-known but little understood [173]. 

TCNQ is reduced to the monoanion or dianion by the electron-reservoir complex [Fe Cp( -C6Me6)] depending on the stoichiometry and the respective redox potentials can let predict the same results with TCNE. 

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