Multi-center electron transfer

We begin with a summary of the standard single-electron rigid-bridge model for electron transport [1,2], and then describe effects that arise from bridge dynamics. We next examine issues in multistep multi-center electron transfer. The closely related problem of two-electron transfer is then discussed. Multi-center and multielectron processes are of great relevance for ET in DNA, proteins, and catalytic reactions. 

The best molecule mimicking multi-step electron-transfer processes in the photo synthetic reaction center so far reported is a ferrocene-meso, meso-linked porphyrin trimer-fullerene pentad [Fc-(ZnP)3-C60] in Fig. 13.16b, where the C60 and the ferrocene (Fc) are tethered at both the ends of (ZnP)3 (R = 46.9 A)... 

Figure 4B shows the set of 31 produetive and 21 unproductive electron transfer reactions in multi-redox center oxidoreduetases with structures available in the Protein Data Bank (PDB). Both productive and unpro-duetive reactions have statistically indistinguishable distributions, which are in turn indistinguishable from the arbitrary protein paeking distribution. Thus Nature has not generally selected protein heterogeneity to assist pro-duetive and hinder counterproductive eleetron tunneling. 

Peculiarities of the N2 molecule make it necessary to use special means of electron transfer to and inside the active center containing the substrate. The mechanism of the catalysis in protic surroundings, at least for dinitrogen reduction, presumably necessarily includes coupled one-electron transfer from an external electron donor and multi-electron transfer to the substrate coordinated in the polynuclear complex. The coupled electron transfer helps to activate and reduce the difficult substrate dinitrogen at ambient temperatures. 

Bridging ligands capable of binding more than two metal centers are less studied. Some examples were highlighted earlier. Polymetallic ruthenium complexes have been synthesized using the tetra-bidentate ligands (62) and (63) as multi-electron transfer agents.181,182... 

Rearrangement and exchange reactions of main group organometallic compounds offer a rich variety of processes important both to the inorganic and organic chemist. These processes have provided examples of electrophilic substitution, electron transfer, and of concerted multi-centered processes, all of which are of both theoretical and practical importance. 

There are several approaches to mimic functions of photosynthesis, and each has been reviewed extensively. These include the design of molecular assemblies duplicating functions of the photosynthetic reaction center [13,14], development of photosensitive materials capable of light harvesting and inducing electron transfer processes [15-17] and tailoring multi-component systems where light-induced electron transfer processes drive... 

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