Diffusion effects, electron-transfer reactivity

Interfacial electron-transfer reactions between polymer-bonded metal complexes and the substrates in solution phase were studied to show colloid aspects of polymer catalysis. A polymer-bonded metal complex often shows a specifically catalytic behavior, because the electron-transfer reactivity is strongly affected by the pol)rmer matrix that surrounds the complex. The electron-transfer reaction of the amphiphilic block copol)rmer-bonded Cu(II) complex with Fe(II)(phenanthroline)3 proceeded due to a favorable entropic contribution, which indicated hydrophobic environmental effect of the copolymer. An electrochemical study of the electron-transfer reaction between a poly(xylylviologen) coated electrode and Fe(III) ion gave the diffusion constants of mass-transfer and electron-exchange and the rate constant of electron-transfer in the macromolecular domain. 

A large number of these values are close to the diffusion limit. This is not actually very surprising since the coupling of the aryl radical with the nucleophile has to compete with quite rapid side-reactions, if only its electron-transfer reduction, for the substitution to be effective. When taking place homogeneously, the latter reaction itself at the diffusion limit and the parameter that governs the competition is Anu[Nu ]/Ad[RX]. This is the reason why a discussion of structure-reactivity relationships is necessarily restricted to a rather narrow experimental basis. 

The ultimate way to control diffusion and reactivity with redox partners is to restrict diffusion by anchoring a portion of the redox molecule and allowing essentially only one-dimensional diffusion. This is effectively the case for the FeS center of the bc complex, which has a mobile head group with a surface-exposed FeS center, but also a transmembrane anchor secured to the membrane portion of the bc complex. This severely restricts the range of motion ( 16 A) but perfectly controls the problem of guiding electron transfer. Diffusion over this distance should be on the submicrosecond time scale, much faster than the catalytic turnover of the complex. In a certain sense, this restricted diffusion has properties that lie between unrestricted diffusion and fixed redox cofactor chains a sort of chain with moving parts. 

The proximity of the diffusion limit also inhibits a detailed discussion of the data in Table 7, but a significant difference to the substituent effects discussed in Section III.D.4 is obvious. Whereas the reactivities of terminal alkenes, dienes, and styrenes toward AnPhCH correlate with the stabilities of the new carbenium ions and not with the ionization potentials of the 7r-nucleophiles [69], the situation is different for the reactions of enol ethers with (p-ClC6H4)2CH+ [136]. In this reaction series, methyl groups at the position of electrophilic attack activate the enol ether double bonds more than methyl groups at the new carbocationic center, i.e., the relative activation free enthalpies are not controlled any longer by the stabilities of the intermediate carbocations but by the ionization potentials of the enol ethers (Fig. 20). An interpretation of the correlation in Fig. 20 has not yet been given, but one can alternatively discuss early transition states which are controlled by frontier orbital interactions or the involvement of outer sphere electron transfer processes [220]. 

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