Multi-electron mechanisms of redox reactions Switching molecular devices

6 Multi-electron mechanisms of redox reactions. Switching molecular devices 

There are a considerable number of reactions in which the products contain two electrons, more than the starting compounds, and the consecutive two-step one-electron electron transfer process proves to be energetically unfavorable. In such cases, it is presumed that the two-electron process occurs in one elementary two-electron step. An example of a two-electron process is the hydride transfer, when two electrons are transported together with a proton. BH4, hydroquinones and reduced nicotinamides are typical hydrid donors. A specific feature of quinones is the capacity to accept and then to reversibly release electrons one by one or two electrons as a hydride. Therefore, quinones can serve as a molecular device, which can switch consecutive one-electron process to single two-electron process. 

Another possible two-electron mechanism involves the direct transport of two electrons from a mononuclear transition metal complex to a substrate (S). Such a transport alters sharply the electrostatic states of the systems and obviously requires a substantial rearrangement of the nuclear configuration of ligands and polar solvent molecules. For instance, the estimation of the synchronization factor (asyn) for an octahedral complex, with Eq. 2.44 shows a very low value of asyn = 10 7to 10 8 and, therefore, a very low rate of reaction. The probability of two-electron processes, however, increases sharply if they take place in the coordination sphere of a transition metal, where the reverse compensating electronic shift from the substrate to metal occurs. Involvement of bi- and, especially, polynuclear transition metal complexes and clusters and synchronous proton transfer in the redox processes may essentially decrease the environment reorganization, and, therefore, provide a high rate for the two- electron reactions. 

The multi-electron nature of the energetically favorable process does not evidently impose any new, additional restriction on its velocity. Within a coordination sphere the orbital overlap is effective and, therefore the resonance integral V is high. The strong delocalization of electrons in clusters, polynuclear complexes in clusters and polynuclear complexes reduces to a minimum the reconstitution of the nuclear system during electronic transitions and, therefore, provides a high value for the synchronization factor. 

As an example, a four-electron transfer from two M+n metal atoms in a binuclear complex may be visualized  

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