Electron reservoir

Figure C3.2.15. Schematic diagram showing (A) electron hopping between electron reservoirs via empty states of an intervening bridge, (B) tunnelling, and (C) hole hopping via filled states of an intervening bridge. From...

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). 

Several strategies based on porphyrin derivatives have been developed to reach this objective. The first approach involves the utilization of dimeric cofacial metallic porphyrins adsorbed on the surface of an edge-plane graphite (EPG) electrode.18 An 02 molecule was expected to be coordinated to form a fi-02 bridge between the two metal centers allowing subsequent scission of the O O bond by reductive activation, while the dimeric structure acts as a four-electron reservoir. 

Muller, A., Das, S.K., Kogerler, P., Bogge, H., Schunemann, V., Krickmeyer, E. and Preetz, W. (2000) A new type of supramolecular compound molybdenum-oxide-based composites consisting of nanocapsules with encapsulated keggin-ion electron reservoirs cross-linked to a two-dimensional network. Angewandte Chemie-Intemational Edition, 39, 3413-3417. 

Compound 14 is diamagnetic and represents the first tetrasodium-dication cluster that is stabilized by two sterically congested silyKflu-orosilyl)phosphanide counterions (see Section II,D). It has been also independently synthesized through sodium consumption of 13 in the presence of styrene as catalyst in 24% yield. The electron reservoir of the Na) cluster can serve for reduction processes, that is, it reduces Me3SiCl to hexamethyldisilane (see Section II,F). The fact that 14 is intensely yellow, and not red or blue as observed for Na-loaded zeolites (28), suggests that the residual metal electrons are probably much less delocalized. 

The Fermi level can be visualized as the surface of an electron reservoir sometimes called the Fermi sea . If for some reason an empty level exists with energy lower than EF, it is filled immediately. Also, if an electron occupies temporarily a level above EF, it will readily fall back to the Fermi level. 

In the open molecule coupled to an external electron reservoir, which fixes the system chemical potential, they combine the minimum-energy responses in the system number of electrons and the remaining nuclear coordinates to a unit displacement of Qs. The associated MECs,... 

Let us now examine these electronic-nuclear coupling effects in more detail. The moderating exchange of electrons between the molecule and its hypothetical electron reservoir determines the effects of the electronic-nuclear coupling in the open molecular systems. Let us assume the initial electronic and geometric equilibria in such an initially open system p° = p.rej and F° = 0. The LeChatelier stability criteria of these two (decoupled) facets of the molecular structure requires that the conjugate forces A/jl(AN) or AFS(AQS) created by the primary electronic (AN> 0) or nuclear AQs > 0 displacements,... 

Here, the relaxed softness matrix Srel groups the equilibrium, fully relaxed responses in the subsystem numbers of electrons, following the displacements in the chemical potentials of their (separate) electron reservoirs, the relaxed geometric softness matrix... 

Although the role of the P-cluster is not yet clear, it is believed to act as an electron reservoir originating from the Fe-protein in readiness for the redox requirements of FeMoco. 

The main point of transformation in Scheme 1.29 is that the central cobalt atom eventually becomes an electron reservoir. 

While donor substituents assist in ortho and meta protonation, acceptor substituents direct protonation of the primary anion-radicals to the ipso and para positions. It should be emphasized that water treatment of the naphthalene anion-radical in THF leads to 1,4-dihydronaphthalene. Notably, the same treatment of this anion-radical, but o-bound to rhodium, leads to strikingly different results. In the rhodium-naphthalene compound, an unpaired electron is localized in the naphthalene, but no protonation of the naphthalene part takes places on addition of water. Only evolution of hydrogen was observed (Freeh et al. 2006). Being a-bound to rhodium, naphthalene acts as an electron reservoir. The naphthalene anion-radical part reacts with a proton according to the electron-transfer scheme similar to the anion-radicals of aromatic nitro compounds (see Scheme 1.14). 

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. 

Fig. 20 Schematic representation of a two-terminal device. The scattering region (enclosed in the dashed-line frame) with transmission probability T(E) is connected to semi-infinite left (L) and right (R) leads which end in electronic reservoirs (not shown) at chemical potentials Eu and r, kept fixed at the same value p for linear transport. By applying a small potential difference electronic transport will occur. The scattering region or molecule may include in general parts of the leads (shaded areas) (adapted from [105] with permission Copyright 2002 by Springer)...

Iron complexes with organic ligands are widely used as such reservoirs. For example, the stable 19-electron complex cyclopentadienyl benzene iron(I) is a clean electron reservoir that has a sufficiently negative oxidation potential and can be easily made, stored, and handled and can be weighed accurately (Alonso Astruc 2000). 

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